How to calculate tire size?

The tire spec will give you overall width at its widest point . Backspacing , as was posted before, should be between 4 1/8- 4 1/2" . If the backspace is too much , tire hitting on the inside , shims are available . If the tire is hitting the outside you have to change rims . 14" rims are more forgiving than 15" rims but the selection is aweful . 15" tires are getting difficult . 16",18"& 20" are the rage today but look wrong on an A-body , IMO , out of proportion . I would start with a 235/60R-15 rear and same up front . If you want you can go 205/70R-15 front . Bear in mind that a 70 series tire is taller. I would do a mock up first . Get steel rims and . Put junk tires on them and see if you like it . Everyone's taste is different . Good luck .