If its just for looks, 275's out back and 245's or 255's up front would give you kind of that look. But since you have bullitt rims, you could find a set of 10.5 deep dish bullitt rims for the rear that should match the set you have now. You should be able to go up to 315's without issues after that. Or check C.L., in my area I always see guys selling staggered rims for Stangs. Might find a pair cheap.
But for best performance, 255's should be more than enough on stock rims.
Edit: if your planning on svt or saleen rims in the future, slap some 255's on now, and save up for the set you want. And in most cases, a good quality tire is going to grip better than a cheaper wider tire.
2000 Mustang GT-Former giggle gas addict
1990 Mustang GT w/ Vortech V3
***resident gun idiot***