Many mopar rear brakes off 8 1/4 axles, 8 3/4 axles and 9 1/4 axles will swap. It seems like some of the newer (late 70's+) 7 1/4 axles may also swap. (The A body 7 1/4's won't fit)

The earlier 8 3/4 axles used brakes with a different drum design than the later 8 3/4's. These had a center bolt holding the drum on the axle. I don't believe the later brakes will swap onto these axles easily.

If we knew what year and model car the 8 3/4 axle is from, some better advice could be given on the swap.