Putting a longer rear axle will make the truck go faster, but will also make the first 2 gears longer, the truck will be more sluggish in town, it may also end up taking a looooong time reaching that new top speed... Also, be careful, how long a gear ratio can the engine pull (enough torque?).
OK, now that you are a bit more confused, my recommendation would be to look into an overdrive trans, a modern 5-speed or original OD: this way, you have a fairly short 1st gear for getting the truck moving, and also a longer 5th speed to go faster.
On a lighter side, you realize that if your truck goes faster, it will be more noisy... Do you really want to be noisier? ;-)
