Eric there are a lot of variables in tire and wheel fitment. On my Dart all of my measurements were taken after I made sure the stock A body 8 3/4 with stock 10" drum brakes was centered side to side then I took all my measurements to center the wheel and tire combo between the leaf spring and fender well ( my car has a dana 60 in it now that was narrowed to the exact width of the 8 3/4). If you add B body drum brakes or disc brakes your going to need more backspacing. When measuring the wheel to center the tire, 1 inch has to be added to the width of the wheel to get the correct backspacing for centering( backspacing is measured off the back rolled edge of the wheel a 7 inch wheel is 8 inches wide from outside rolled edge to outside rolled edge). Eric on your wheel picks, if I put them on my Dart the 7 inch with 4.5 backspacing and 255/60 are centered. If I went to an 8 inch wheel on my Dart I would need 5 inches of backspacing to recenter the 255/60 tires and I have yet to find that backspacing in an 8 inch wheel other than custom made. A 275/60 will fit if you move the springs against the frame rails and pick up that 1/2 to 3/4 inch of space and then remeasure for backspacing to center the tire in the new space.
