Tire clearance

Like I said in my other post, a 7 inch wheel with 4.5 inch backspacing(mine were Weld wheels) with a stock A body axle with stock drum brakes. Makes all the difference between a 255/60/15 tire fitting, without rubbing and rubbing on the outside wheel well and lip.
I ran those same 255 tires on a set of 15x7 police wheels with 4.250 backspacing and on hard turns and big dips/ bumps they would rub on the outside wheel well, on my 66 Dart.
One of my buddies has a 67 GTX(not an A body) but have the same tire clearance problem. He has 15x7 with 4.250 backspacing Magnum 500 wheels on the GTX, he first mounted a set of 275/60/15 drag radials on the wheels(because he had them) and they contacted the outside wheel well. So he bought a set of 255/60's fit perfect. Back to the 275/60's I took a set of my 15x7 4.5 bs Weld wheels over one day and we mounted the 275 on them, that tire/wheel combo fit his GTX as good as the 255 combo.
Don't know what it is about those 15x7 4.5 bs Weld wheels, but that added .250 inch of backspacing always seem to be the perfect wheel for all the tight wheel well Mopars. At least in the ones I have dealt with, with stock width rear axles and brakes.
If you have modified your axle width or put disk brakes on the back, then all bets are off.