Well, I had a 295/50/15s on 15x8 inch wheels with 4.75 inch backspacing on my 69 for a while. I had to roll the fender lips for clearance, but had plenty of room. Too much backspacing with a 15 inch wheel and you end up with a clearance issue with the lower shock mounts.

I'm currently running a 295/35/18 on an 18x10 inch wheel and have a little room to spare. Just do your homework and measure, measure, measure!!!