MPG experiment....1976 Dart Lite

If I recall, it was up to 36 MPG highway with the 4 speed od. I would LOVE to be there. For now, increase in steps, and then when it appears the tune is limited to RPM of the 3 speed, step to the 4 speed OD.
I essentially want to be able to document, 3 speed to 4 speed overdrive on a well tuned ride. I see people who step to the 4 speed OD, but I wonder how much MPG is left, where maybe the 4 speed is masking another possible improvement in tune.
Hopefully have the carb back this weekend, for an update next week....?