My 78 F250 (400\4 speed\Edelbrock intake with Holley 4 barrel\4.10 gears) typically got 10-12 mpg around town or highway, with a load or without. The gas mileage dropped off at speeds higher than 60 or so. You will not likely improve the gas mileage by going to a 302 or swapping the trans to an AT.

I've recently swapped to a Q-jet on a rebuilt 400 with a different cam so I'm hoping for an increase in mpg but even then it may only be a couple mpg.

A high school kid with little income is going to have a hard time keeping that truck on the road.