Do you have the factory wheel flairs on it?? If so, then without trimming, 265 (I think) is as big as you can go. I have a set of junk 285s on stock wheels, my t-bars are slightly cranked, and they will rub turn to turn on flat surface. Start flexing it will rub more.
Just go down to your local tire shop and ask them to throw a 285 on a wheel like yours and them put it on your truck. See how it fits and if it will rub. Thats what I did when I put 33's on my stock Dodge not long before I got rid of it. The tire place wants to sell you 4 new tires. They will be happy to test fit a few for you.