The B-body 8 3/4 rear is the way to go on this swap (1963 to 1970 B-body)
The A-body 8 3/4 has too many issues not the least of is the smaller bolt pattern and the limited availibility
The B-body rear is the right size to use the spring relo kit and will most likely have the bigger brakes and
upgraded third member. (A-body rears most often have the 741 case instead of the 742 or 489 which are
stronger)