Ok, here's what I've read about max tire size: Vintagewheelworks.com (http://vintagewheelworks.com/fguide.html
) recommends going with no bigger than 225/50/16 front and 245/50/16 rear on a 68. Crunching some numbers would give us 24.86 inch diameter and 8.86 width on the fronts, and 25.65 diameter and 9.65 width on the rear wheels. I think these might be conservative.
I read in Chevy High Performance Magazine that they recommend going no bigger than 245/40/17 front and 275/40/17 rear (with 5 inch backspacing front and 5.5 rear). Calculating these out would give you 24.74 inch diameter and 9.65 width on the fronts with 25.66 diameter and 10.83 width on the rears.
On the rear wheels you might have to relocate the shocks inboard to get that kind of width. I think the thing that is important is looking at what your total diameter, your total width, and what backspacing you will have, not so much if you have 17 inch wheels and 40 series tires (or any other combo).
What I put on my car that works so far is 245/45/17 front and 275/40/17 rear so that both front and rear had about the same diameter (25.68 and 25.66 diameters) with widths of 9.65 inches front and 10.83 rear. This size might be pushing it for some people, so be careful (I can almost fit a finger between the rear tire and rear wheel well on the inside). I don't have too many miles on this combination, though. I've got 17x8 (0 offset) front and 17x10s (6mm offset) rear.
What I would do is read what other people have posted works and what doesn't. I read somewhere that a guy had 255/40/17's that rubbed (25.03 diameter and 10.04 width), so I figured that it would be hard to get a 10 inch width tire to work on the front (though some people may have gotten them to work).
I too had a previous Camaro that the wheels would hit in the bumps (going fast though underpasses, etc.). This car had the front springs that were shot (too soft). If your suspension is in great shape, you might be able to go with pushing towards the upper ends of these limits. If your suspension is older, you might want to be more conservative. If you find out you are hitting on the outside fender, you can take the chrome trim off and/or roll the lip of the fender with a baseball bat (from what I've read).
Your 235/60/15's have 26.10 diameter with 9.25 width. If you can figure out where you're hitting, you can narrow it down if they're too tall or too wide, or if your suspension needs to be fixed somewhere.
Hope this helps! [img]tongue.gif[/img]