Hy chebbydog, welcome, you have to remember that when those trucks were on the road and new 50 m.p.h. was probably the highway speed. You have many options to increase speed, taller rear tires, a rear end swap using a lower (numerically) rear end ratio (higher top speed), installing an auxiliary (brownie) transmission between the existing transmission and rear axle, or installing a transmission with overdrive as the top gear, if you search these different options you will find a lot of information. Keep in mind these trucks in stock condition were never engineered to travel at todays freeway speeds, brake upgrades would also be a smart idea, hope that helps.
