You'll probably rub up front but in the rear i think you would be fine... Maybe need some harder springs to keep from bottoming out. I have never seen one with 18's on it but I am just assuming. I love to see pictures though if you do it.
I know there are people that have run them. My 17X9 rub on a full locked turn, so your deffinetly going to have rubbing in the front. In the rear, if you took the quad shocks off, you'd prolly be ok.
had 18x9s on my 87 lx and my 90 gt, 245/40s and 255/35s, no prob with either, the lx needed +18mm and the gt used +24mm. just get the correct offset for YOUR car. all stang differ alittle.