Im running 315/35/R17s in the back and I have no clearance issues and I've seen in the Mach 1 forum (and on M1R) that several people are lowered and running 18s. You should not have any problems at all. On a side note though, I have 275s up front and they rub on full lock only, if I would have gotten 265s I would have probably been OK but really how often to you turn your wheels to full lock?
I know I'm not running your exact setup but I thought I'd toss my info out there so take it as you will.
Assuming the correct offset is used, you can run the 315's in the rear.
ditto....i was running the same 315/35/17 setup with my mach lowered on your same springs. i had no issues. even though it looked really close