[puddleglum]

Welcome to the forum. It's possible that rebuilding your engine is part of your problem mileage wise. If you tightened everything up in the rebuild, it may take some time to break in again, just like in a new car. Also, the cracked header may be effecting the O2 sensor readings. May not be enough to through a code but enough to reduce mileage. Just brainstorming with you. Did your mileage drop after all that you've done or is the quest for better mileage something recent?
Automatics are harder in general to hypermile due to the limited driving techniques at your disposal. I found it worse when I did a lot of in town driving. Wondering if your commute might be fighting against you.