8 3/4 axles 8.75 left and right differences

Moparparts said
E-Body rear is narrower than a B-Body rear. I have a few E-Body sized ones available

It all depends on what year B-body rearend your talking about like Rob said below. The older B-body rears are narrower than an E-bod rear. Those are the ones most people use in an A-body.

crackedback said
71-74 B are the only b rears that are longer than an 8.75 e body rear. EVERY other 8.75 B rear is shorter than any 8.75 E rear.

You can use whatever you like, just need to size wheels appropriately.