what size SBP wheels can we fit under a rear 1963 fender?

If you are lucky 15x8 may fit. I put 15x8 with zero offset and 245/60R15s on one car and they cleared with 1/2" clearance both sides and 1/2" to the springs both sides. No rubbing with plus 1" rear springs. Tried the same wheels and tires on one of my other cars and they were about 1/4" from the fender well on one side and about 3/8" on the other side. Would have rubbed for sure. Guess the fenderwells are a bit wider on one and narrower on the other. So with luck 8" could fit, but don't count on it.