What axle modifications did you make?
Yes, the center register is smaller on the stock A body 8-3/4 axles and the backing plate/drum flange offset is different also.
Any 65 up large bolt pattern rear drum sets will fit. Get the whole set-up if you can, drums, backing plates, hardware. I have a few sets of late 70's 10x2-1/2" brakes from 7-1/4's and 8-1/4's that fit the 8-3/4's.
I just noticed the earlier post - no, B-body axles are too long to fit and they have a taper preventing them from being shortened & resplined.
I believe he was talking about getting the brakes from the B body rear end, not the axles.
I bought mine from Moser, first thing they asked, was if I wanted A-body offset, or B-body offset.
I bought the A-body offset axles, which allowed me to keep my original backing plates, brakes, and drums. Just had to redrill the drum for the bigger 5-lug pattern.