Slant 6 balance

Unless the engine is a "Frankenslant" that has been assembled with mismatched parts, I would suspect the clutch. If you can swap in another runner, that would be an easy way to troubleshoot the issue (I know an engine swap sounds drastic). To have your current engine balanced, you would need to pull it anyhow, completely disassemble and take the reciprocating assembly to a machine shop. The balance job will be $200 or so, plus a complete gasket set, rings, bearing inserts and timing chain since you're in there anyhow. This could eat up $500 really quick with you doing all the labor.

Later model slants don't have spark plug tubes. Not sure what the change year was.

If you're driving along at the speed that the vibration happens, and you pop it into neutral and coast letting the engine drop to idle, does the vibration go away?