He has already swapped the converter and flex plate. I am not trying to be offensive to him in any way but rather he is beginning to chase ghosts as my old German boss would say. (IE: chasing his tail.)

He did one thing that MAY have alleviated the problem. I would go back and repeat that test to confirm or deny its affect. Then go from there

I will say I had one years ago that I did everything short of pulling the motor and tearing it down. Even did new bearings. NADA, never did find it, but I put 40K + miles on the car before selling it to a friend. 5 years later he said it had been the best car he ever owned except for that 67-72MPH vibration