wheel/tire size (71 dart swinger)

No not at all.

Guys run the same tire in the 15" wheel all the time!

So 255/60/15 is what you need if you are using 15's. 7" Wheel with 4.25 backspacing.

Yep, you can fit 15x7s (4.25" BS) with a 255/60/15 tire, at least a MT anyways, now all was really good untill i had my green bearings installed on the axles, they didn't press them on far enough, the tires still fit, but theres less room in-between the lip molding on the DS, nothing my dye grinder can't fix, but amazingly, it still doesn't rub, but man is it close. Mad dart, those tires look naked under there, why didn't you install some 15s? I hope those tires hook for you, my MT DRs sure do lol.