This thread seems to be losing a bit of focus. It also fails to mention the goals and what car it's intended for.

All else equal, 318 will surpass a 400 in mileage.

Also, I'm not buying the deeper gear might get better mileage. May for the 5 to 10 seconds it takes to accelerate from a light but once you are at a constant speed. Lower rpm equals better mileage.