What is the largest wheel's I can put on a 1974 dodge dart 360 sport

I have a dart sport with an 8 1/4. I want to run custom offset rallys but was unsure of backspace judging by pics on the silver dart sport I can probably get 15x10's with 4" backspace and be right on the edge of inner quarters which is what I want! Even if I have to trim inner lip in quarters.:D
That should work, the 3 3/4 back space still had room from the spring's.