What engine would you build for a Ramcharger?

Apples to Apples comparison.
My dad bought a 1978 power wagon, 360 4-speed, 3.55 gears brand new, he still has it, 14.5 mpg almost every tank, he has documented every gallon of gas put in that truck since new, it now has about a 250K miles on it.
I just sold a 1978 power wagon 318 4-speed, 3.55 gears it got 14.5 mpg but was gutless, added a LD4B intake and 4 bbl, mileage stayed the same, and it was still gutless.
I put a built 360 mag in it, power went up considerably and the mileage stayed the same.

Honestly if I was driving 100 miles a day, I would be spending my money on a 4cyl front wheel drive car from any manufacture, just find something clean and used. Wear items like tires etc, are cheap.
The difference you are paying for fuel, would most likely pay for the car in a year.