285/65/18 will be fine, you could fit those easily without any type of a level, you sure you don't want to step up to at least a 33" tire? I know that tire is wider but a little taller tire would look better in the wheel wells.
On stock wheels your main concern will be the width of the tire b/c of the offset of stock wheels, height wise you should be able to run a 34" tire but with stock offset the width would probably kill you. Too bad you are not local (my assumption) I would seriously consider selling you my tires as I would really like to step up to a 34"
I have a 3" level with 295/70/18 terra grapplers (34.71" tall) my aftermarket rims are +18 so I did have to modify my body mount to stop the minor rubbing at full lock. Your factory offset would be fine on your wheel well I think, it's gonna be the UCA clearance that'd probably be your problem.
TundraTalk.net forum offers the most comprehensive collection of Toyota Tundra information. Find discussions on the Toyota Tundra Diesel, TRD Pro, SR5, lift kits, towing capacity, classifieds, and more!