Speaking from experience here..... I put a 6 pack set up on the 318 in the 69 Fury ragtop that I had for 17 years. All stock original long block, except I put a 68 340 cam in it. Stock convertor and 3.55 gears. That land yacht would still pull 18mpg if I kept my foot out of it.. not a rocket off the line, but top end out the yingyang.