1974 Dodge Dart Magnum Swap

I think at this point, after re-reading all the details about your build and the vibration, and if you have already ruled out ignition timing, I would retest at same steady RPM with any/all belts removed, sitting stationary in park again. If it still shakes, get the balancer triumphman mentioned in post #48, Professional Products Part #80012. Install it with no weights first, then if it still shakes, try adding the Magnum weight. Another option would be installing a different style factory Magnum balancer than you have, if you can get it cheap. The newer one has a serpentine pulley integrated, so I am sure you don't have that one yet, but the Professional Products part will work with pulleys you already have, and it will allow you try three different balance setups with one part.