3000 stall convertor and 2800 highway cruising

i think any converter is going to kill mpg. sure its going to build heat but with a sufficient cooler it will be fine for a long time. if you want to build a car for roadtrips, mpg and still make some power your best bet is a manual trans with od. next would be an auto with overdrive, but i think you really need oen that uses a lockup converter to be efficient, and the converter choices are sort of limited.
i only did the power tour kickoff but it was still a 3,000 mile trip in my big block duster, with a t56 and 3.73 rear i managed 19mpg on the highway, this trip was just 2 weeks after completing the engine and trans swap. i didnt even get to mess with the carb much.