Below is measurements from my go to chart. Others should chime in for its
accuracy.
The other issue is pinion centering with transmission. Mopars offset the engine to one side. Measure CL of trans output to a frame rail and use that to determin offset to CL of the pinion.
8 3/4" Rear end widths, drum-to-drum:
A-BODY
'66-'72= 57 1/8” (or 57 13/16” for BBP axles and brakes)
