Depends a lot on where you drive - surface street stop & go, or highway.
Accelerating kills mileage as the weight of the car increases. On the highway, tall gearing & better aerodynamics help mileage.
Depending on how many people and how large they are, I'd be thinking A-body if it's large enough for your needs. Low 20s should be pretty easy with the right setup. I think the best I ever saw out of my Dad's 383 C-body was 17MPG highway.