You can order the gears through most any place online, 4 Wheel Parts, Summit,Jegs, etc. You will need deeper gears if it's an automatic than if it is manual shift. With an automatic and 33's you'd probably want 4.10 and with 35's probably 4.56 and that's assuming it's an auto with O/D. If it's a manual you could drop each of those suggestions down one selection, numerically speaking. For example 33's you'd use 3.73 and for 35's use 4.10. Having the correct gears with the big tires will be important in making it "wake up" since you don't have a powerhouse motor in there.
Oh, and nice truck by the way, I LOVE the old square bodies. I may even have some stuff laying around you might be able to use, I'll check the garage.