I am trying to find out what is the largest pair of tires and rims that I can put on a 67 Plymouth Belvedere II with the stock wheel wells. I don't want to enlarge the fender wells. I would like to know the backspacing to use. I have a set of 15x8 American Racing Sprint wheels with 4.5" backspacing on 235x70R15 tires. They fit well but would like to put larger tires in back.