i would go along with the TQ swap mentioned above. smaller motor doesn't necessarily mean better mileage - my 78 D100 only got about 9 mpg with the 225 /6 1bbl and a 4 speed. the worn-out 440 in my car was getting 13 at that time. i always wanted to put a 360 or 318 in my truck to get better mileage, thinking the slant 6 made it too under-powered, thus working harder to get moving