We have a 70 Cuda with 15x7 Ralley fronts and have just fitted the 15x8 Ralleys to the rear.
The 15x7's run 245/60/15 BFG Radial T/A's and fit on the front without any problems (the fit is also determined by ride height of course)
The new 15x8's run a 275/60/15 BFG Radial T/A and we had issues with the tyre on one side rubbing on the inside lip of the rear quarter panels.
Im not 100% on the what the backspacing is as the wheels came with the car, but we had the centres moved out 10mm (approx 3/8") on the rims to enable the wheels to sit closer to the spring. We didnt want to roll the lip up on the rear quarters.
The new measurement from the inner leaf spring to the tyre at the rear is about 10mm and the quarter panel inner lip to the tyre is now approx 10mm also.
Not sure if this helps.