There are a couple of 5 speeds that could be swapped but the price for a rebuild is high and finding a used one can be difficult. The most desirable is the NV4500, but there was also the Getreg 5 speed that was offered. Another option is a Clark 5 speed, but it's over one hundered lbs heavier, and even harder to find then the 4500. You may luck out and find an automatic with overdrive.
Depending on the tire size you have and the gear ratio in your axles, you may or may not pick up a couple of MPG extra. The question is wether it will pay for itself in fuel in the long run. If you plan to keep the truck for a long time, it maybe worth it but then again, you may not recoup the cost in saved fuel.
Ed