The 3.90 gears don't fit the closed driveline rear end. The simplest conversion to get better highway speed with a minimum of modification would be to install the 3.55:1 rear end gears sold by Mother Trucker, who is a member on this site. A rear end bearing and shim kit will also be required, as well as the new ring & pinion gears. That keeps the closed driveline and your original transmission in place. The gear swap is not cheap, and will probably require the assistance of someone familiar with setting up rear end gears properly (few shops and even fewer hobbyists do) but the result is an original-appearing truck that's better able to keep up with traffic. Just be sure the brakes and steering are in good shape to be able to handle the increased top speed safely. The cost of the gear swap will probably be less than making the switch to an open driveline, different transmission, and rear axle change.
Jerry
