yeah, I was measuring last night between the fender and bed side. I hung a string down from the inside edge of the fender. It seemed like I could get 10" tires in there, but that may be pushing it, especially if one side travels up and it starts to angle. I think 9" would be safe.
The part I'm most worried about is the back space. I didn't tub it, that will be a project for some winter most likely. It's tough to get a real hard number for back space with a tape. I think 3 1/2" should be correct, but I don't think I'd feel comfortable buying expensive wheels and then finding out I'm 3/8" off or something.

I'll check out that link now. Thanks for the input.