18x7.5 will tuck perfectly 18x8 at most without rolling in fenders. My friends has Gold VolkRacing 18x8 GT-N's 3 piece awesome rims. Perfectly drops and tucks in that is the most I would reccomend. Anything wider you will have to modify the fender panels accordingly.
Here's a pic of it I took at the show.
Just makes sure you get a lowpro rated tire. Hope this helps.