Daughter needs Gas mileage from 273 commando

I would change the heads to the 88-91 318 heads 4323302...the fast burn, swirl combustion chambers will help burn efficiency immensely. My friend had an '88 Diplomat with a high mileage engine and these and it easily got 15mpg city with 2.76 gears, a 999 Torqueflite and a very heavy foot.