Agree with 76GMC1500. I had a 305 in an '84 Trans Am, and it moved out pretty well. Good general purpose engine - not as impressive as the 350, but it did a good job across the RPM range. It would break the tires loose (posi), and still get down the road.

Mileage will be a problem with 4.10 gears. My '65 F100, with its original 240-I6, three on the tree, and 3.70 gears, never got over 13-14. With the FE (bored 410) and C6, and the little gas tank behind the seat, it's a short-range vehicle. Low gears and bad aerodynamics.

The overdrive auto trans should help, but the 4.10s still are low. My wife's 2000 Expedition has the aerodynaimcs of a brick, and will get 14 at 70, and 19 at 55. 261ci engine, OD auto, 3.73 gears, 17 inch wheels (stock, with 265-70 tires).

Slow down a bit and the mileage will go up. Basic physics - it's an exponential curve for fuel consumption as speed increases. Even with higher gears (lower numerical), you won't gain as much as if you run slow. Besides, you see more that way...