Fewer cubic inches requires less fuel regardless of how efficient the rest of the build is. If you can keep the cid down while increasing the efficiency of the burn, that is the best answer.

440 in my one ton crew cab did just fine with torque in reserve and mileage that was most often seen in much lighter cars. 4:11 gears with 33" tires and a 727. I'd guess its weight to have been around the 5k range, give or take a couple hundred.