2500km roadtrip in my 70 Duster

My current setup is a mild 360 (rebuilt about ten years ago, balanced, stock 340 cam, Edelbrock 600, cast iron 340 intake & exhaust manifolds' and a full TTi dual exhaust), 904, and an 8-3/4 with 2.94 gears. I usually run a 3.55 with a sure grip. Even with the highway gears, fuel 'economy' is somewhat lacking. Oh well, I didn't buy the car to get good mpg.