It just depends on how deep you want the wheel to look. The best method is to cut the wheelwells out of the way, order your wheels and tires, set them in the car with the proper clearance between the quarter panels and framerails, and then measure between the wheel mounting surface. That measurement is the width of the assembled rear end that you will need in the car. Anything can be made to work with custom backspacing, but the final look might be compromised with wheels with alot of backspacing.