Rear Disc Brake?

Does anyone know what vehicle I can get junkyard rear disc setup for an 8 3/4?
I have a 74 RR.
I have also read about upgrading to cop car front disc brake. What size brakes are these and what vehicle came with them?

I have heard 90's model grand cherokees rear rotors and calipers will fit an 8 3/4 with little mods , but i have never done it .

The 75 Imperial brakes will fit but not without a few mods. They are very rare and finding parts can be a problem. I have a set and will put them on only because I had them for so long plus I have the machine tools to make/modify whatever I need to make them fit. If I was doing this today I'd probably go some other route, like the aforementioned Jeep stuff. I'd also add that putting rear disks is a lot of effort for little gain on the average street car, but there is the cool factor!

The late model front cop brakes are 11.75" and that's what I'm putting on my 69 RR. The spindles are taller than the STD B body but you can use the big bearing A body spindles as a substitute. I plan to use the tall spindles but have the tubular upper control arms from Firm Feel, which take the taller spindles in to account.

Hope this helps.