I just put M/T ET Street Radials 255/60/15 tires with 15 x 8 (4.5 inch backspace) steel wheels on my daughter's 67 Coronet.

I would have to say that is as big a tire that I could safely put under there with everything being stock. 265/60/15 might barely fit but it would need to have a different backspace and probably custom wheel spacers. I was not willing to take that chance.

Right now I can barely get my index finger between the fender lip and the tire.