all you need is a spare tire that is the same height as the rear tires. That way, if you get a flat, you can put the spare on and not have to worry about the posi burning up on you. If you have a front flat tire, your pickup might just look kinda tilted, but that wont hurt anything.
18's and 20's are good combos, but havent seen great rims that big on a nice look AD yet. Oh yea, you might need to tub the box a little bit for the 10" wide rims out back.
Drew