Best possible mileage?

Fuel economy is often lost, just in the getting up to speed.
A half a dozen accelerations from a dead stop with a hiway-geared, 3500# combo, is gonna drop your mileage significantly; no matter how lean you make the cruise circuit.
And if you saw on the throttle, the Accelerator pump is not helping

In the quest for fuel-economy, in a given combo; a dash-mounted, dial-back, timing box, coupled with a distributor that has serious adjustability, will be your best friend, and a high-amperage ignition system or Multi-spark, is your second best friend.