Best NON-ELECTRONIC engine/transmission for highway cruising

[chewiebarker]

My family and I like to travel by car a lot and we do so, typically, in our 2012 Suburban LS. That thing glides on the highway, just set the cruise control, sit back relax. Effortless, easy, 75-80+mph cuising.

Obviously, in contrast, my 71 Burb, 350/350, on the highway is torture. The hard grinding of that drivetrain, just to cruise at 60, makes it impossible for us to take any long distance trip in it. In fact, it tops out somewhere between 65and 70 - and it is hard getting there! Now, I know that my engine could use a refresh, but I am fairly certain that even if it was getting all the HP it could, the combination would not do much to improve the experience.

So, my question is, WITHOUT GOING TO an ELECTRONIC CONTROLLED TRANSMISSION, which combination of engine/trans is going to get me some better long distance highway performance? I am willing to put the right components in, I just don't know how to get the gains in performance I want and stay out of electronic controlled transmissions. I don't want 'new tech' because I already have that in my 2012 Sub. Thoughts?