'65-67 B-body 8.75 will add about 1" per side of width. You do not need to relocate the spring perches. You can drill the 7.25 shock mounts. I believe you can even use the 7.25 backing plates/brakes if you have 5x4.5 bolt pattern. Only thing will be shortening the drive shaft.
I made this video to show the swap. Complete rear ends can be bought for about 200-250 bucks, often including drums/backing plates.
