A 315 is a fat *** tire! But it's common among mustangers, I'm rollin on 285/18 w/2 inch drop all the way around and I could easily go wider. I think you'll be fine. Maybe someone who's actually got'em will chime in and set your mind at ease.
I had my old one lowered about 1/3 inch (not too sure but my fingers didn't fit in between the tire and fender) from the fender In the front and about an inch apart in the back. I know for sure the springs were cut because it was the stiffest ride ever. But I had 265 in the front and they should hit the fender wall when i turned too tight and in the back from the looks of it the guy had to bend the metal next to the tire so he could fit 275 on the back