On the front, you cannot really determine what size wheels and tires you can run until you install the suspension you are going to use. To try to "anticipate" is merely asking for headaches and extra expenses. I would "guess" however that 18 x 8 is a lot of wheel and tire in the front of one of those trucks... at least if it is sitting down where it should be. OK, I like 'em low.
For my '55 Chevy pickup, I have 15 x 7 GM ralleys with 225/60R15 and 15 x 8 with 255/70R15 front and rear respectively. They are not on the truck yet (still under construction), but I know they will fit. Later on, I can see how much more room I have to get a better feel for fancy (expensive) wheels and tires.
Obviously I can't say everyone, but I think most trucks that have 18 inch or larger wheels on the front are using air bags. I don't know if you are or not, but just wanted to mention that.
Dennis W. Parks
Author of automotive how-to books