There was a bit of a discussion on one of the other threads about fitting aftermarket calipers. Part of my mods is upgrading the rear brakes so I thought I'd post a few pics of how to build an adaptor bracket for a radial mount caliper.
Radial mount calipers are much easier to adapt to a custom setup than a lug mount caliper. Start with a block of alloy about the right size (I used a 32mm square section). Mark out a center line on the block as all your measurments will be taken from here.
With the caliper sitting in place on the disc resting on its pads measure the distance from the mounting face on the caliper the center of the bolt holes in the the hub. Mark a line on the block at this distance. Now you can drill the holes in the block for the bolts that attach it to the hub. The spacing needs to be equal either side of the center line. You can measure the distance between the lugs off the old caliper as it's easier to measure on a hole that's not threaded.
Once you have the block bolted to the hub you can double check the pads are centered on the disc face. If they are out to far you can machine the face of the block down to get the pads in the right position.
Next you can get the caliper centered either side of the disc. Measure the width of the slot int the caliper. Mine are 31mm wide and my discs are 26mm thick. This gives me 2.5mm clearance either side of the disc. work out the distance from the inside edge of the caliper to the center of the mounting hole. Add your 2.5mm clearance and to this number and mark that hight on the block relative to the face of the disc. Now you can drill and tap the holes in the block where the caliper will bolt to. Work out the distance between the mounting holes (mine were 154mm) and mark that on your block in relation to the centerline. If the caliper mounting holes and holes to bolt to the backing plate are both referenced to the center line then the caliper should be concentric to the disc.
Here's a pic of my roughed out bracket. The caliper is mounted in the right position and everything is square. I've just got to machine off the excess material and make the bracket look pretty.
Cheers