I have the 73-76 Disk brake conversion on mine. I thought that pushed the tires out a .25 inch which hurt clearance. What brakes are on yours?
I've tried 235/60/15 on a 15x7 4.25 backspace and I had some slight rubbing lower front wheel lip. I ran 225/60/15 on 15x7 rallies with the stock 4.25 backspacing for years though.
If you have stock SBP drums or disks I think you probably could get away with 235/60/15 on a 15x7 rallye with 4.25 backspacing (stock)
15x8 have 4.5 backspacing that helps, but the 1" wider width will put the centerline of tire still out .25 inches compared to a 15x7 with 4.25 backspacing. So a 15x8 4.5 bksp will give you less room in front than the 15x7 rallye
Some cars will very depending on past collisions and production variablity.
