Thanks everyone, sounds like I might be able to make it work (with some work on the suspension tuning, which I figured would be the case). The car should be set up pretty good to hook with the coil over/double adjustable shocks up front, caltracs adjustable shocks out back and the split mono leaf/caltrac slider so it'll just me a matter of me getting it sorted out.

IROC78, I know plenty of guys have run the 325/50/15 with this setup and my measurements seem to confirm that. the 315/60/15 has a slightly wider section width though (despite the tread width being narrower). M/T says the section width is only wider by something like .3 IIRC but I think the height combined with the added section width would get me. I think the issue would come from where the inner wheel houising tapers into the 1/4 panel lip, the added height would put it much closer to that tapered section.

I could be wrong and it might fit, but from what i've heard the 325/50/15 is a very tight fit, so I would assume .3" wider and a whole 2" inches taller in diameter would be too much. I could be wrong, but it would be so close I think I'd need a tire for mockup to make a determination and I don't know if it's worth looking for one when i'm almost positive it won't fit without rubbing pretty bad.

If anyone has successfully done it though, please chime in as that might give me some motivation to find a mockup tire and see if it'll work on mine.