Transmission delimma

[scotts72]

I have a question and I'm sure I will get a lot of suggestions, I'm just looking for the best solution so I only spend the money once. I have a 68 with a SB350mild cam and TH350 trans, the Rear end is a 3:73 Posi, my dilemma is I live in Texas and the speed limits are mostly 70 so that means 75-85 here. I am wanting to change out the Trans to make the truck more efficient and enjoyable to drive instead of hitting 3500 to 4500 grand to just keep up on the highway and avoid being run over. Just wanted to see maybe which trans is the best replacement and has been reliable.