Absolutely nothing wrong with a 255/60R15 tire on a 7" wheel, although that is the minimum width wheel according to BFG. The problem is the axle/wheel combo doesn't center the wheel/tire in the wheelwell. I'd bet that some massaging of the inner wheel tub would allow to run what you've got. It wouldn't cost you a dime, and you'd still be able to run those great looking tires. There's nothing goofier than small tires on the back of one of these old Mopars.