This is probably what yuor looking for. This write up is from Matt Bobbit's website.

6. Rear Disc Upgrades: The rear Caddy caliper conversion:


Here is what I have done for the rear: I bought some custom rear brackets that use the Ford ThunderBird Turbo Coupe 87-88 rear rotors that are 10" vented 4 lug units and Cadillac rear calipers from a 80-85 Seville. This is a very similar way to make a conversion like what is offered by Master Power Brakes, ECI and BM for hotrods. These calipers have built in parking brakes, a 54 mm piston available in steel, aluminum and phonemic, and are readily available in junkyards. You can get the brackets from SteelTechSoutions Inc or search on Ebay for "rear disc" and you will see them listed. Use yoru stock 87-93 4 lug axles for the stock rear track or the 87-88 Turbo Coupe/93 Cobra 4 lug axles if you want a wider rear offset.

Here is the parts list and prices that I got to give you an example of what is needed:

-Custom rear brackets
-'87-88TC/'93 Cobra rear Rotors; 4 lug
-Rear calipers from 85 Cadillac with all lines, springs, pins, etc: The casting numbers on the calipers are 020 and 021. This is an easy way to determine if they are the correct units.
-New Pads
-2 x 3/16" couplers - or use the TC rear softlines with adapters
-Adjustable proportioning valve
-FMS M-2450-A plug for the stock PV
- Brake fluid
-cable ends to shim the stock parking brakes -- or use the FMS M-2809-A cables
-rear soft lines from 87-88 TC
Why Cadillac Calipers? Well for one that is what the brackets are designed for and you can get the brackets. I know that it is hard to get SSBC to sell you just the caliper brackets. You can use your stock axles. You have the choice of 4 or 5 lug. There are a lot of 80s Cadillacs in the junkyards to pull the calipers from. You can modify your stock parking brake cables to work with the calipers. The stock brake lines bolt directly to the calipers. They are a large piston compared to the Ford rear calipers. You can run 4 lug plus 5 lug with the same calipers. You can run different axle offsets. Lastly, they use the same pads as the front GM Metric calipers. So you have a wide selection of pads making them are cheap and plentiful in parts stores!

***If you are using Cadillac ElDorado/Seville rear calipers there are some important things you should know. One of the biggest advantages of a disc brake system is the fool proof self adjuster. Not so with this rear GM system. The rear calipers adjust off the parking brake. The parking brake is incorporated into the caliper. You must set the parking brake every time you park the car . The rear caliper pitons utilize a one way clutch inside the caliper piston. When the parking brake is applied the clutch senses when there is .030" or more clearance between the friction material and the rotor on the inboard side. When there is more than .030" the clutch turns inside the piston adjusting it out keeping the rear brakes adjusted. If you do not set your parking brake every time you will start to lose brake pedal (low and spongy) and the adjuster mechanism will not work any longer. Also: never use rebuilt calipers on the rear because the rebuilders use the old pistons and the pistons were the reason the caliper failed in the first place. If you do, get a lifetime warranty on them like at Autozone!

I was going to do this to my car, buy ended up going with Cobra rear Brakes, if your interested in doing it this way, I have the brackets, Brand new set of 4 lug cross drilled and vented rotors along with a brand new set of steel piston caddy calipers, still in the box for all of it. Only thing I dont have are the cables which you can buy from Summit Racing.