Checking the coupling with feelers gauges gets the engine and prop shaft in line, as the cutlass bearing end of the shaft can't 'flop about'.
How old are the engine mounts? They could have lost their stiffness.
I have Beta38, PRM120, R&D coupling, cutlass bearing. My vibrations and noise improved tremendously when I had the prop reconditioned by Crowthers. Not cheap but worth every penny. My shaft was worn, and I also replaced it and the bearing, along with the engine mounts, but it was the prop that made the difference.
Richard