You missed a little something. The torque converter.
Under heavier loads, some vehicles of mine and some trucks get better mileage because the converter is working and not slipping. I have found this loaded / unloaded truck example working on my vehicle and others have told me about there experience’s.
Yeah, torque converter can definitely impact mileage. In the case of @motorpirate's Durango any my Dart we're locked up on the highway, but the OP will definitely have to consider TQ.
Years ago my car used to have a 904 (or maybe it was a 998 or 999?) out an 85 5th Avenue. It had the mechanical lockup, where it would lock up as soon as it got into 3rd gear. For @gagembassett's max fuel economy build that would probably be the best transmission that could be easily installed (i.e. doesn't require any modifications.)
I also didn't mention the thermostat. I'm running a 180° but if I really wanted max economy I would go with a 195° stat.