Someone has there 2002 roush stage 2 at the ford dealer next to my house, they are 18's, a dish style tho, he has 295/35/18 on the rear and it looks sick. I wish i could have those rims wih those tires on my gt. as for the front he runs 265/40/18 fronts. a very nice set-up. good luck
Those are going to look very nice on you car. I wish you luck on getting them to fit well. I always hear horror stories about 18's on a fox.
Sorry I don't know what size tires you should use, but I think you should keep the sidewall as small as possible. Try to find tires that are the same diameter as your tires now...