Help me confirm my measurements and back spacing thought

So it's an original A-body width rear end. Just big bolt axles installed?

The inner wheel well curves in. When the car leans to the side, the top of the tire tilts in relative to the body.

I have a 68 Dart that has the same wheel lip as your car (same wheel lip molding). I've got 245/60/15 15x7 rim with 4.25 backspace and small bolt axles. It barely touches on the outside and inner wheel well top and the springs. Springs stock location.