As I recall the A-body specific 8.75 has different spacing from axle flange to backing plate than other mopar rears. There are several posts that address what parts should fit though.
Only the SBP axles/brakes have a different flange offset.
BBP brakes and axles have the same flange offset for all applications, so you can use BBP backing plates from any 7.25/8.25/8.75 or Dana 60 with BBP axles.
What you can't do is redrill the SBP axles to BBP and use stock BBP brakes, or use stock BBP axles with re-drilled SBP brakes. If you re-drill the SBP axles you have to use SBP brakes and redrill the drums, and if you use BBP axles you need to use BBP brakes. The only exception to that is if you buy aftermarket BBP axles with the SBP flange offset, which you can do. Why you would want to spend the money on aftermarket BBP axles and then hamstring yourself by re-using your re-drilled SBP brakes I don't know, but you can get aftermarket BBP axles with the SBP flange offset.