The answer is
IT DEPENDS
In another recent thread on PT
http://www.pro-touring.com/showthrea...t-of-68-Camaro
Bret said the ridetech car with uncut fenders has has 18x9.5's with 275's and 5.5BS
In this thread,
http://www.pro-touring.com/showthrea...etech+Tru+Turn
Ridetech says the 48hr camaro has 18x10 with 275 and 5.75bs
The question becomes the other mods. The 48hr camaro has cut fenders, and shorter than stock body bushings. It more than likely also runs more negative camber than anyone without Ridetechs tire budget would run on the street. Negative camber can be the dealbreaker because the inward tilt of the top of the tire makes a big difference when you are talking about barely clearing and enough scraping to be a pain in the a$$. For example, this camaro,
http://www.rhoadescamaro.com/build/
is being built to an SCCA legal class, and cant use many of the parts protouring cars have access to. He is running in excess of -5 degrees of camber with 265's on 18x9's with stock suspension. Works on the track, but that much neg camber would equal short tire life on the street, not to mention it almost looks like something is broken with the tire tilted so much. The 48hr camaro doesn't use near that much, but even with -2 degrees, tire life will be shortened considerably.
The only way you are going to find out on your car is to get a 275 and mount it, using spacers to get it dialed in. If you take the spring out so you can check clearance at all ride heights with the car on jack stands, you should be able to do it with washers instead of getting a bunch of expensive spacers.
Let us know what you find out