Well, you didn't say what size wheels..but I assume 17's or 18's. I also didn't notice that you are lowered in any way by your signature, so I'm guessing you're at the stock ride height. Eitherway, I've never heard of anyone having a problem fitting 17's/18's w/ 315's on a Stang...should look really nice actually!! As for mustangtuning.com, they are one of the more popular sites, so it's not too surprising to see them out of stock, kinda sucks though.
The wheels should fit, they will look fat on there too!!! The reason why mustangtuning is out of stock is because everybody and there moma gets the bullitt rims.
I've got this combo installed on my '99GT. Originally mine was lowered w/ Steeda sport lowering springs then w/ Eibach pro-street coil-over setup. In both cases the tire corners were scraped and even had the edges shaved (cut) by those sharp fenders. It especially occured when transitioning road surfaces diagonally (ie: from a level roadway going up my driveway which is on a hill. When hitting the hill diagonally 1 tire is pushed into the fenderwell and rubs the fender & gets chewed up). If you do get this setup you might as well plan on rolling the fenders or those expensive tires will get eaten alive in no time!!!! I'll take a pic for ya tomorrow if I can find my camera.
315s cause no problem at all, we have an incoming ETA on our computer system saying early July on the wheels, for what its worth...
We also sell the 315s incase you didn't notice.
I wish we carried those new BFG 18 inch 315s, I've not really seen any other affordable brands in that size. Does anyone here know if 315/35/18 cause any problems?