1981 Ford E-Series Van mpg

it is hard to make a box(van) get gas mileage. Other than keeping it in tune and maybe a gear change not much. these usually fall into the 12-15mpg range and not much else can be done with them. At least one as old as yours.

The best ideas would be interchange the old heads for pedestal rocker heads; @ 85 heads(v-8), or head (300-6). Bronze guides superior ro cast iron.
The engine compression should be @ 200 psi, and switch to a dist cap with brass contacts. Low resistance plug wires and an MSD blaster SS coil.
Use Lucas oil treatment and a good synthetic 10/30.
Good mileage requires precise valve geometry and precise preload. Vacuum reading in prk should be @ 22in./hg, warm engine.