Backspacing still doesn't make sense to me.
Ok, well I have Weld Draglites. 15x8s with 235/60R15s on them, and I have lots of clearance. I know I can get 255/60R15s on with NO problem.
I'll get you the backspacing when I get home (Come to think of it, someone else was asking me about backspacing here).
I've got a 75 Dart Sport, so it should all be the same.