Tuning a DD 383 for optimum mileage / 1/4 mile performance

Those who bluntly say Performance and Mileage don't go together are fairly short sighted, as they are exactly the same thing!
The goal in both cases is to make the engine run more efficient in the rpm-range of choice.

First step I would make is to go digital with the timing advance curves.
You can only do so much with springs and weight in a mechanical distributor. With a digital ignition you can tailor every rpm-range and every vacuum-range (work-level) of the engine.

Focus on getting an engine to run more efficient in all rpm-ranges and you'll get rewarded by better mileage and performance.