There are two ways to make your truck's top speed higher. Change the gearing (Tranny, Tires, Differential) or increase the RPM range of the engine. (or both).
Safely driving faster means better brakes and suspension.
To make that truck safe on the freeways without permanently altering it, you could get a new ring and pinion set for the rear differential, radial tires, tube shocks, put a set of sway bars on the front and back, and put on a front disc brake conversion kit.
3.55 for the rear diff would be nice, 3.08 would be better, though harder to find. Maybe you could find a car torque tube with 3.08 and swap just the ring and pinion into your differential. With 3.08 gears you could happily cruise at 65.
None of those changes are anything you couldn't undo in an afternoon except the ring and pinion, which might take a saturday.
If you want to drive the truck around, the most bang for your buck safety wise and drivability is gonna be: Radials, Brakes, Tubeshocks, Turnsignals and Ring & Pinion gears. Another option that wouldn't require permanently altering your truck is a Saginaw 4-speed, like the conversion offered by Patrick's. Patrick and his son PHillip are both very helpful and will answer any questions you have.
http://www.patricksantiquecars.com/order.htm If you just want to show your truck and trailer it around, don't touch it.
