Largest Wheel Sizes

[bslyth]

I have looked around the forums for a while but can't quite find a definitive answer (I see many responses and answers).
My Q45 is envisioned on 18 inch rims, that are as wide as possible. I was hoping to get some 240 or greater tires which is around 9 inch.
What are the widest wheels the Y33 can take? I don't mind if front and back are slightly different (I can understand steering issues for wide rims and would rather not modify the wheel well except possibly fender flaring for a wide-body appearance).

Most wheel companies recommend 8.5 as the greatest width for the Q, but I feel like that isn't the true limit. Any help would be greatly appreciated. Trying to price my build and start when I finish school.