First and foremost, a set of spacers are needed to widen the calipers.  The calipers need to be widened by the same amount that the rotor is wider than OEM.  In general, we are talking about replacing the 1/2" solid rotor with a 3/4" vented rotor so we need to use 1/4" spacers to widen the calipers; however, it is plausible to use thicker rotors still -- 7/8", 1", whatever -- and suitably thicker spacers would be required.  In general, widen the calipers by at least as much as the rotor is widened.

This is a picture of one half of a caliper -- the inboard half of the left side caliper, to be precise.  The mating surfaces on either side at the top of the picture is where the spacers go.  To make things really pretty, you need to cut out your spacers to the same shape as those mating surfaces so it looks really smooth when it goes together.  However, for practical purposes, the cutout doesn't really need to be so precise.  You should, of course, make sure the spacer is at least as large as the mating surface of the caliper to give the mating joint a solid "footprint".

Since those mating surfaces are less than an inch wide, you can use rectangular bar stock that is one inch wide and the desired thickness to make your spacers from.  1" x 1/4" rectangular bar stock is readily available.  You can also make the spacers from 1/8" or even 1/16" stock; just make more of them and stack them up.

Drill the two bolt holes first, then use the bolts to hold the spacer against the caliper while you mark out the outline.  On the edge facing the rotor, you need to make sure the edge of the spacer doesn't interfere with the rotor itself -- but it's actually perhaps an eighth of an inch or farther away, so you probably don't need to cut it too close.  The end of each spacer pointing towards the pads can be left quite long, just make sure it doesn't interfere with the installation of the pad retaining pins -- but they are more than 1/4" away.  The spacer cannot possibly interfere with the pads themselves, since the spacer is 1/2" narrower than the rotor.  The outer end on the end with the handbrake calipers must be short enough to avoid interfering with the bronze fork installation; the outer end on the other end (lower end in the car) can be almost any length.

The spacer edge that is critical, believe it or not, is the upper edge in this picture, the one facing away from the rotor, the one facing forward in the car -- especially the spacer on the lower end of the caliper as it sits in the car, the one on the non-handbrake end of the caliper.  Especially the one on the right side caliper.  This surface just barely clears when removing or installing the caliper in the subframe.  Make sure to grind it down to flush with the outer surface of the caliper itself in this area.

Palm happened to have some 1/4" steel plate laying around (doesn't everybody?) so there was no need to purchase any bar stock.  The stuff was good 'n rusty, so a little steel wool was in order.  The completed spacers looked like this:

These spacers don't cost any money to speak of, but they do take some time to drill and cut out.  It'd be nice if somebody would set up a machine to stamp spacers out of sheet metal 1/8" of an inch thick, so we could just buy whatever number we need and stack them up between the caliper halves.

The next items needed are the 3/8"-24 bolts to hold the calipers together.  Don't use the originals, they're too short and you want to make sure to engage enough threads to hold the halves together securely.  The original bolts are 2-1/8" long -- but they're actually a bit too short to begin with!  They could have been a quarter of an inch longer originally before you even start talking about spacing the caliper.  For 1/4" spacers, 2-1/2" length would be good; they'd probably work OK with 3/8" spacers too, although 3" would also be acceptable and 2-3/4" would be ideal.

Go with Grade 8 or "alloy steel" bolts; this is not a place for scrimping on cheap bolts.  It doesn't really matter what type heads you find; socket head allen bolts might look cool!  The original bolts have no locking plates or lockwire because they are fairly long bolts; the length will provide enough "stretch" when torqued to ensure they won't vibrate loose.

You're going to need to do something about the bridge pipe, the pipe that connects the two halves of each caliper.  It obviously is bent for a particular spacing between the fittings, and you've just increased that spacing by the thickness of the spacer.  If the spacers are only 1/4", you can actually bend the original tubes just a bit and make them fit.

In this picture, the upper bridge pipe is the original shape.  The lower one has been suitably bent to fit a caliper with 1/4" spacers in it.  Note that the bending needs to not only address the additional distance between the ports, but it also must result in a configuration that will clear the corners of the caliper and the handbrake pivot pins.  It is advisable to have a caliper assembled with spacers on hand when doing this bending.

The original tubes can be bent to connect to a caliper with 3/8" spacers in it; this is the procedure used with Gran Turismo Jaguar's kit which uses 3/8" spacers.

If you prefer, you can have some new bridge pipes made.  This isn't difficult; pre-made brake lines are available in auto parts stores, totally straight -- you have to bend them to the shape you need.  Make very sure you get the correct type fittings; they need to have the correct threads (3/8"-24) and they need to have the correct style of tip.

The handbrake caliper pivot pins are secured with a locking plate.  Providing a positive retention scheme to make sure the pins don't unscrew is obviously a good idea from a safety standpoint, but the OEM locking plate won't work once you have spaced the caliper wider.  One idea would be to drill a 1/16" hole crossways through the head and apply lockwire.  This will work, but it might be a bit of a challenge without a drill press, and you don't want to screw up that pin because you'll have to wait on shipping to get a replacement.  If you do go this way, you might consider using some flat washers so the head isn't tightening down directly on the bronze fork.

Another idea, obviously, is to make locking plates.  This actually isn't too difficult, but it will take some patient cutting -- and next time you have this thing apart, you'll have to make two more!

The bronze fork on the handbrake caliper pins is a bit more of a challenge.  You can, in fact, just saw the original one in half and bolt it in, and everything will appear to work fine.  The only problem is, you might as well just leave the bronze fork out.  It's supposed to work as a "deformable spring" that pulls the handbrake pads away from the rotor when released.  If you cut it in half, it becomes a useless part.  The pads will simply ride against the rotor.  The end result is that the handbrake pads wear quickly.

Jan Wikström cut his bronze fork in half and brazed in a section to widen it.  Workable.  Another idea may be to cut a slot in each half of the fork from the center of the cut edge all the way across to 1/8" from the bolt hole.  Then when assembling, install a piece of steel that fits snugly into this slot all the way across, thereby holding the two halves aligned.  Either a locking plate or washers under the pivot pin heads will hold the steel piece in place.

It might be an idea to start with four forks, and cut each one off center so that you can braze the long ends together to make forks the correct width.  You might even be able to cut each one in such a way that they lock together when assembled, and brazing is unnecessary.

Palm used an idea gleaned from a picture of parts in a kit from Terry's Jaguar Parts -- but apparently they have changed their kit, because Brian Schreurs now reports that Terry's kit doesn't include any parts for the handbrake assembly, assuming you will simply omit the handbrake in entirety.  Anyhow, the idea is to make a locking plate with an edge folded down to provide the requisite support for a bisected pair of bronze fork halves.  The fork halves therefore cannot turn because they run into this lip.  Palm fabricated such a locking plate -- but he didn't provide any locking tabs on it!  Instead, he drilled the heads of the pins for locking wire (Palm has a drill press).  This way, each time the assembly comes apart new lockwire can be used but the plate holding the bronze fork halves in place can be used over and over.

Assembling with this hardware involves using two halves of a bronze fork on each caliper (cutting a new bronze fork in half is recommended).  Install the bronze fork halves first, making sure that the tines of each half of the bronze fork are correctly inserted in the holes in the handbrake caliper and that the handbrake adjuster is backed out far enough that neither handbrake pad contacts the disc.  Position the plate on top of the bronze fork halves with the lip pointing down toward the main caliper.  Install the pivot pins (flat washers are recommended to protect your homemade plate) finger tight.  With the parts so assembled, operate the lever on the handbrake to get the adjuster to begin taking up the slack and applying a load on the tines of the bronze fork halves.  As they get loaded, the bronze fork halves will try to rotate around the pivot pins until their sides jam against the lip on your plate.  When the bronze fork halves have been so loaded -- hopefully still without the pads contacting the rotor -- tighten the pivot pins down securely.  Then continue to operate the handbrake lever until the adjuster has taken up all the slack and the handbrake pads are positioned very near the rotor -- but the tines on the bronze fork halves are pulling them a tiny bit away from the rotor whenever the lever is released.

Brian Schreurs reviewed the assembly of a 6.0 litre TWR:  "on this car, they sliced the fork in half and secured the halves by drilling small holes through the fork and into the caliper, and holding them down with cap screws."  Excellent plan; for security, you could use cap screws with lockwire holes and lockwire them together with the pivot pins that you have drilled for lockwire.  Or, if you have found or made a lockplate to fit the spacing of the pivot pin holes, you could countersink the holes in the bronze fork halves, install flush-head screws, and install the lockplate over top of them.

Another idea is to forget tapping the holes you drill in the calipers and using screws.  Instead, press-fit some pins into the holes so that they protrude just a tad less than the thickness of the bronze fork.  Again, having a lockplate over top of these pins would provide security, but it's probably not necessary here if you've found another method to secure the pivot pins themselves.

Don't get carried away operating those levers by hand.  Just get one click per swing.  When really loose, it's possible to move it far enough to get two clicks, but it's also possible to get the clicker to pop out of place inside the adjuster -- meaning you'd have to take the thing back out and open it up again!  You don't wanna do that, I'm sure.

The pins that hold the main caliper pads in place have to be replaced because the originals are now too short by the thickness of the spacer.  This is a really easy job; in fact, there are three ideas presented here.  The first is the way Palm went, which was to make new pins from really long 1/4" bolts.  The original pins are 2-5/8" long under the head, so the 1/4" bolts need to have a smooth shank at least that long plus the thickness of the spacer.  Palm bought 4" long 1/4" hex head Grade Nil (there's no need for strength here) bolts at Wal-Mart, where they come two to a pack (with nuts!) for less than a buck.  Saw the threaded end off, then chuck the rest of the bolt up in a lathe or drill and grind the head down to something that looks nice.  Note that of the four holes these pins must fit through in each caliper, one is recessed, so the head must be ground down at least far enough to fit into that recessed hole.  Finally, drill a 1/16" hole for the clip the same distance from the headed end as on the original, and you're done.  Can be done in just a few minutes.

In this picture, the upper pin is the original.  The middle one is the longer one made from a bolt; at the bottom is the type bolt it was made from.

Another idea is to purchase a length of 1/4" steel rod (available at hardware stores), cut it into suitable lengths, and drill two holes in each for clips to hold them in place.  You need the second hole to make up for the lack of a head.  Drill the two holes located so that the clips will be just inside the openings in the calipers, with enough space to allow it to rattle a little.  The prime disadvantage of this method is that you're going to have to find four more clips!  The advantage, though, is that once the clips are removed, the pins can come out either direction.

Finally, yet another solution is to buy some genuine Girling brake pad retention pins that happen to be longer.  They're not too hard to find, either -- they're about 8 feet away at the other end of the same car!  Yes, the pins used at the front end of the XJ will fit perfectly; they are part number 12798.

After Palm thought about things a little more, he decided that installing the pin retaining clips in the vicinity of the caliper dust boots -- where they were originally -- may not be wise, even though the problem of these clips damaging the boots appears to be more of a concern on the front brakes.  It is easy enough to drill new 1/16" holes out at the end of the pins and put the retaining clips outside the calipers.

The handbrake calipers only present one concern with widening, and that is the link that connects the two caliper halves.  This is part of the adjuster; one end is threaded so the adjuster can operate by turning a threaded cogwheel on it, and the other end is pinned so it cannot rotate.  It looks kinda like a long screw with a slotted head.  When the calipers get 1/4" farther apart, these links needs to be 1/4" longer -- or do they?

The adjustment range on the existing links does seem to be long enough to deal with 1/4" spacers -- but just barely.  To properly assemble the handbrake system, the adjusters need to be adjusted quite a bit loose and the bronze fork tines configured to hold the pads away from the rotor.  When assembled, the handbrake levers should be manipulated back and forth repeatedly until the adjusters pull the pads against the rotor and preload the bronze fork tines.  To assemble with 1/4" spacers and new handbrake pads, the adjuster needs to be out on its last thread when installed!  But that's OK, because adjusting it to pull the pads against the rotor will thread it in about another 2 or 3 turns, providing enough thread engagement to hold it securely under load.

If you are going with rotors significantly wider than 3/4", the original links won't do.  It should be easy enough to find replacements, though; both are right hand thread (the two adjusters are not mirror image of each other, they are in fact the same part flipped over, and everything except the caliper itself is interchangeable right to left side).  A socket-head alloy steel screw would be good; chuck it up in a lathe (or a drill) and grind the underside of the head to form a ball shape like the original, and then use a hacksaw to cut a slot in the top for the cotter pin.

There is another alternative, though, and that is to use used handbrake pads.  Since the adjuster must screw together farther to take up the wear, it gains thread engagement as it goes.  Obviously, if you are using new handbrake pads, you can take a grindstone to them and apply a little wear.  You should trial-assemble the handbrake calipers without the bronze forks and mark how you'd like to grind on the new pads, since they need to be ground on a taper -- which will probably mean you'll need to grind on one end only.