The SM319 transmission would be the easiest bolt in swap. The main case is the same as the SM318 trans that originally comes in these trucks. Plus, they are very common. The trick is not to limit yourself to just looking for a truck variant of this trans, the passenger car trans is the same, and will directly swap.
While it is true that the function of the overdrive is to lower engine rpm at a given speed, it does have the secondary function of allowing higher speeds while keeping the rpm down. I have no problems running one at highway speeds (70+) with stock suspension, steering, and brakes.
