Getting them to go faster can be accomplished in a number of ways, and many folks on the board have used various methods with success. It must be remembered that once you start making modifications for speed, upgrades to the braking system should be strongly considered. Also, the power requirement to maintain higher speeds goes up dramatically and you might find your stock engine struggling. All that said, there are a number of options. Although I'm not that familiar with the 4100 size: There are several 5 speed transmissions with an overdrive that can be installed, with varying amounts of modification needed. The rear axle can be changed with one from a more modern truck to obtain a better axle ratio. It might also be possible to add an auxiliary transmission or overdrive unit to the truck between the stock transmission and the rear axle. I'll leave the model numbers to another member with swap experience. I don't always have luck with the search feature, but there are past posts available covering many options.
Someone will chime in with more options and some specifics about how to accomplish the various mods.
