Mileage is not different, apples to apples comparison.
My dad bought a 78 power wagon, 360, 4-speed, 3.55 club cab short box, new, recorded every gallon of gas put in it, 14.5 mpg is what it got.
I bought a 78 power wagon, 318, 4-speed, 3.55 standard cab short box, used off the original owner 49,000 miles it got 14.5 mpg.
Both trucks totally stock, including tire size.
The 360 has more power, gets the same mileage, pulling a slightly heavier truck.
People here need to quit spreading bad info that the 360 gets worse mileage than the 318.