I had this over in the general posting section but it might be better here. My son is working on a 1966 Plymouth Satellite hardtop. we are trying to determine how much tire and wheel we can get under it with butchering it up. Back in the day I put G70/14 Goodyear Polyglass GT's under the g/f's 1967 GTX with stock 14x6 wheels and it was a pain.

I run 245/60-15's on a 15x7 steelie with 4-1/4" backspace in the rear on mine. This is the perfect wheel, in my opinion, as it centers up perfectly. The 245/60's fit really well, with about 3/4" on each side clearance (just enough to get your hand up inside the wheel well to clean the tires when you wash the car). If you're looking to have reasonable clearance and not have any rubbing, I think mine is the optimum setup. However, if you're just looking for the widest tire that will possibly fit, You could go a little bigger. I'm sure 255's would still fit pretty well, or obviously 70 series tires would fill up the wheelwell a little more.

I have a 275/60-15 on a 15x7 steel wheel with 4&3/8 BS. It fits good. I do have SS springs though.