Whats the widest tire you can put on the back of a 98 without the tire sticking out of the wheel well?

I put 305's on my 03 Z06. It should fit the same on your 98.

Actually, the Z06 is a little wider than the standard model, most people usually use the 295 rear tires from a Z06 on the standard models without sticking out. You can usually find a good deal on a complete set of wheelsand tires from a Z06 on ebay, or the forums. We sell new OE reproduction wheels at very good prices if you're interested, check out the smokin deals section.

Ed