[Steeveedee]

OK, if it still does it warmed up, it isn't a rich mixture and reversion at the exhaust leak. The next thing that comes to mind is carbon tracks in the distributor cap or wires arcing over, since it isn't rhythmic. As has been mentioned, that's way too high an idle speed. See if you can get the idle down. That won't cause this problem, though. You can know that because if you used to gas pedal to get to the same RPM, it wouldn't be doing it. In other words, I'll bet it still does it at idle.

Do you have an AIR system on your truck? I see it's a '67, and that common equipment for California-bound vehicles, and C10 trucks.