My O/T 1-ton dually with a TBI 350 and NV4500 5-speed will do 13 MPG or so at 65-70 on the highway. And it must have a 4.88 or 5.13 in it, you could pull a house off the foundation with the granny low in it. A 2-carb intake would probably help, the most fuel-efficient carb would probably be the little 2bbl they made out of the Rochester quadrajet in the late '70s, if you can find a pre-computer one with the 2bbl casting (I have one here that uses the 4bbl casting, with the secondaries blanked out). That and a gear change to get the RPMs down would probably do it. The rest is just aerodynamics. It sits high and believe it or not the air passing under hurts you some, an air dam or even lowering the front some would help to an extent. That's why land speed cars often run an air dam right down almost to the ground.