I ran 255/60/15 in the rear and 235/60/15 in the front when I used ralley wheels. We tried fitting 255's in the front, but they would rub the body on hard turns so I had to go with the next smaller size that he shop offered.I hear that 245/60/15 fits all the way around and that is what I would try to get if I where you.
Congrats on the purchase and welcome to the forum! :cheers:
If your front suspension is in rebuilt or otherwise very good condition, you can probaly do 255s all around. If you like fat tires, you'll definitely be happier with there than those piddly little 225, 235 things :jester
Look at http://www.corvettefaq.com under "misc." and read "More Bite For Older Sharks (Wider Tires)".
I did this change and I'm running 255-60-15's (front) and 275-60-15's (back) on stock rally wheels with great success. Looks very good IMHO.
The fender/wheel well is shaped differently on 68-69 cars and are more restricted in terms of tire size in the front. I could not get the 255/60/15's to fit before OR after I rebuilt the front suspension on my 69.
If the fenders are not a problem, suspension condition can matter because old rubber control arm bushings and old balljoints will let the wheel move out of it's "ideal" position, and thus it can move closer to the frame and rub more. My 255s rubbed very slightly on the front before my rebuild, I haven't tried again since then, but I will find out when my new Pirellis arive.