 315
315's will fit, though it depends a bit on WHICH 315's you use. All tires have varying widths (regardless of having the same metric size tread width). So, which tires were you considering buying. If you haven't shopped for them yet, be advised there are not many 315 tires in 18" sizes - and they tend to be costly Pirelli or Michelin tires, unless you're looking for track tires or drag radials (which of course have very brief lifespans, though they are very sticky).
It also depends to a large extent on the setback of the WHEELS you buy. Wheels that push out the tire placement away from the car allow you to run larger width tires than wheels that are offset more TOWARD the car.
Finally, the ability to add modest (less than 1/2") spacers can solve clearance issues if necessary, though its always better to avoid these if possible.
I have more info. about the typical GT (vs IRS cobra) setup, since that is my personal vehicle. I run 18x10's with 295's with no problem whatsoever, and I can fit my entire hand betwen the sidewall of the 295/35 WR18 Firehawks and the inner fender well. I have no doubt I could run 315's with the wheels (Mustang Tuning source, Cobra R Replica deep dish bullit wheels) I'm running.
Hope this helps. Figure out precisely which wheels and tires you will be running, and it is possible to compute the offset (this is usually available from the dealer, but ALWAYS available from the manufacturer) and tire width.
__________________ tripleblack
"You can never be free until you let yourself go." | 
