I have the FFR 18 inch X 11 wheels with 315 30's on the back. With 1.25 inch spacers the fit is good, mine is an IRS car and I just got it aligned. You will need to keep the tire diameter close or run the risk of them hitting the frame at the front by the door striker. 11 inch wide wheels fit a 315 tire well, there is room to go wider but I can't say how much.
David W