Generally speaking, it takes a certain amount of power to move a vehicle. The bigger and less aerodynamic the more power is needed. you may very well find that a smaller engine gets worse mileage as it has to work harder to make the power. In a B300 I wouldn't think a 318 would be a better choice. In 78, same ride essentially, Chrysler put the 225, 318, 360, 400 and 440 in them. I imagine any of those would "fit". If I were looking to do a swap I'd think hard about a 400 but mostly I would just optimize the 360.