Rear

I just checked Doctor Diff for you under his axles section. He has the stock A body differentials listed as 52 5/8" from outside housing end to outside housing end. 68-70 B Body is listed as 54 15/16" from housing end to housing end. Please double check my math, but that is a 2 5/16" difference in width. Likely would have to adjust your backspacing by about an inch on each side. That is assuming the axles hang out the same amount on both differentials.

I was told by them that just switching to big bolt pattern in my Barracuda will cause the axles to stick out an extra 3/4", and that I would have to account for that in my wheel backspacing. I just don't know if that is additive to what I said above or not.