Surely a simple roll of the guards is all that is needed if the wheels are wider than 9". Im intending on getting the widest wheels i can afford and am hoping to get them outside the guards.
Don't waste money with 18" they will be to small, when you put the body kit on. I would go with 20" You should be able to go wit 10" in the rear, and you might consider going with an 8" or 8.5" in front to prrevent rubbing.