Clark,
I got your message and will shoot you an email. Good advice here on the forum. I would start this week by looking close at the distributor cap and rotor for tracking, potentially the coil for arcing to the intake, maybe check resistance of the plug wires and separate them, ugly if required, to ensure no cross fire
I do have both an operable 1850 and 3310 if it comes to that, but the vibration at that high of an RPM seems more of a crossfire, scatter, or dropped spark to me, at least without hearing it
I would also be very surprised if it was a balance issue, assuming it's a 390 crank. It takes a bunch of weight to make them shake when everything is zero from the start, unless you have a broken skirt, etc. Although, if nothing else found, you could check crank end play in the car to see if at an early time in the rebuild, you took out a thrust bearing. Not generally a shake, but if things can move around, sometimes they can do funny things