Gas mileage 225 Slant 6

At the minimum You should be getting the low 20's in the highway for mpg no matter what it is
My coronet deluxe 4 dr 225/3 on the tree got 22 - 24 on the highway at 70 mph. 2.94 8-1/4. Volare super six with a/c was 17-24.

The mechanical rolling resistance needs to be at a minimum. Slant sixes are notorious for stretched timing chains. There must be no stretch in the chain or they run for %$#@.
Min operating temp 195
Tune up
Exhaust
Tire/wheel size choices
Lubricants - very key to the rolling resistance or lack there of
Suspension
Gearing
A bit of aerodynamics - you would be surprised....