Looking for 8 3/4 backing plates and U-bolts

you can use 10" backing plates from a 7 1/4 (may need to drill one hole) but aside from that they are the same.

This only applies to the newer LARGE BOLT PATTERN 7 1/4 rear backing plates. The earlier A body SBP 7.25 plates are different. You can also use 8 1/4 backing plates from just about anything including trucks,b/c/e bodies, and of course, any 8 3/4 setup (except sbp A body)