I have 335/35/17s on 17x10.5" rears, and I'm quite certain I could run tailpipes if I didn't love the way my dumps sounded so much.
That's how close to the fender edge they came. I do have rolled fenders, but they were rolled beforehand. I'm pretty sure these won't fit unless you roll them otherwise. Unless you are going with a wheel that has a different offset and backspacing than traditional aftermarket Mustang wheels, I would worry more about the fender lip than the tail pipes. I'm confident that I could get some tailpipes in there without rubbing if I wanted to.
And for schnitzengiggles...