Putting a B Body 8,3/4 rear end on a A Body

Depending on what year B-body axle you have, it is either 1.8" or 4.6" wider from flange to flange than the original axle. So divide that by 2 and add it to your current backspacing to get the wheels in the same spot.

This chart in BBD might help you figure it out.

http://www.bigblockdart.com/techpages/diffchart.shtml