backing plate?

To answer the ops question

Any 8 3/4 B C E body with 10x 2.5 BBP backing plate will work with 10 x 2.5 BBP drums.

Same for 11x 3 etc
Just to refine that a bit....'65 and newer B, and E body with 10 X 2 1/2" brakes. C-bodies only came with 11" brakes. The later A,B, F, M, etc 10 X 2 1/2" setups should work too. Just make sure the backing plate has the 5 mounting holes....most do regardless of which rear end they're on.