I think this has something to do with your inability to read any codes. your computer may be unintentionally taking fuel away from your motor because of some bad sensor readings. Since you can't dump any codes and neither can your mechanic, something is not right and may be computer is not functioning. Try to sort out the ground issue and codes may point you to the cause of overheating and lean condition.
When you did the engine swap, what computer did you use? What harness? Any missing connections? Just some food for thought.
When you did the engine swap, what computer did you use? What harness? Any missing connections? Just some food for thought.