14x7 Magnum 500's on a 68 Dart

Are those available in small bolt pattern, or are you switching to large bolt when you do the 8-3/4 swap ?

To try an answer your question. I'm currently running 15x7 rallys with 4.5" backspace with a 225/60/15 tire, and I have a little over in inch clearance between the tire and the outer fender. Just about 1/2" between the tire and the leaf spring. So I think 4-1/8" backspace should work fine if you dont go too big with the tire size.