What would be the largest rear tire size(radial not slicks) that would fit well on a 88' Firebird Formula without having to tub the wheelwells?

I ran P255/70r15's on a 15x7 rim, and then later, on a 15x8 rim. I had to use a sledge on the inside front of the Rear Driver's Side Wheelwell to make those fit. Now, I'm running P275/60r15's on the same 15x8 rim, and didn't have to do any further wheelwell mods. I also run air shocks to keep the car raised up; otherwise, the wheelwell lip cuts into the tire.

Hm, this message probably won't help you at all!