You can contact the tire manufacturer and ask for load inflation tables. Weigh your truck front and rear and appropriately inflate.
Ford says my F350 needs 80 psi in the rear tires. Sure maybe at max load, but that is hardly necessary unloaded.
33" tall tires is about as large as you can go on those trucks (depending on spring options/engine) without rubbing in the front. If the truck is a diesel a 33" tall tire won't bother you that much, as far as towing and gearing go. I ran 35" tall tires on my '02 with 3.73 gears. It was a 7.3l diesel.
Also if you know anyone with AutoEnginuity they can change the speedo for you. With a 33" tall tire you might find that it is actually pretty accurate.