I've read on other threads on this site that the following things help with gas milage:

1. Two progressive two barrel carbs rather than one 4 bbl, one two barrel or the original 1 bbl.

2. HEI ignition

3. PCV Valve

4. Dual exhuast

5. Lower gearing in the rear differential

6. Transmission with better gearing.

In my truck I'm doing all that and the cost isn't gonna be much more than a pure stock rebuild because the parts all need replaced or rebuilt anyhow.

If your truck is already running, some of that stuff you can do fairly cheaply with patience and putting in some free time at the boneyard. For example, I got my T-5 for under a hundred bucks, including the clutch disc that'll work on the original fly wheel. The rear differential was $100 (a 3.42 10 bolt posi with good brakes) Throw in a gasket, shim kit and saturday making adjustments. Driveshaft was $15 plus u-joints. The HEI gets pulled out of a later v-6 and ground to work in the straight six. Carbs, manifold and dual exhaust are pricey though!

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi