When I did the rear in my 70 Dart, I made it 56 1/2" wide complete. I used the DoctorDiff spring offset kit and ran a 18x8 wheel with zero offset, 255 45 18's. I moved the rear diff back 1 3/8" to center it in the wheel well. I was about an inch from the leafs and about 1/2" from the lip on the quarter panel. I also lowered it with the Doctordiff kit and used Hotchkis leafs, which lowered it a total of 3.5". Not sure how much all of that will help, but may give you some idea of width
