Question after converting a 7-1/4 to 8-3/4

The backing plates are matched to a brake size and axle housing....not bolt pattern. The only ones that will fit have to come off an 8 3/4 rear with 10 X 1 3/4" brakes. That limits the donors to: A-bodies, and B-bodies with 6 cyl. engines and 10 X 1 3/4 brakes. Not a big pool to choose from. Your best bet is to find someone who has converted to the big bolt pattern and has the old backing plates still kicking around the shop.