P.J. about the only way to increase your highway speed is to change your gear ratio. If you have a 4 spd you could change out to a 5 spd overdrive transmission. You could also change out your rear diff to a later model, maybe a Uhaul or something(might be cheaper) . The bolt pattern and width would be your biggest concern. I'm sure there are trucks out there with higher gears than your 65 has.
