Biggest tire under a stock suspension dart????

Be careful with 255/60R15 on an 8" wide wheel with 4.25" B.S. My friend has this combo on his 69 dart and the tires rub on the rolled fender lip. I am running that tire size and wheel B.S but on 7" wide rims and I have plenty of clearance from the springs and fender lip. Some tires bulge more than others. I have BFG but maybe the Mickey Thompson's don't bulge as much giving some more clearance.