The B-Body rear is 3" too wide for an A-Body they won't fit the early A-Body's at all without the tires rubbing the fender wells, and you have to move the perches to fit the spings.
The car in question is a Duster and the b-body transplant has all ready been done by several members here. Yes, the perches could be moved (although I have heard a few people running them as is) and the only thing was getting the correct offset wheels. Also, I believe some of the transplants were done with 66-67 b-body rearends which are slightly narrower than post-67s.