A Body Tire Clearance

Hey all, first time poster here.
I recently acquired a 1974 Plymouth Valiant with a 400 swapped into it. I was wondering what people are doing for rear end widths for maximum tire clearance with the offset shackles from Dr. Diff. I'm going to have to build a custom width rear for this thing to begin with since the car has an 8.25" under it currently.