360 engine vibration

I always like to think Simple. You are good with the 4 cylinder opposed VW engines, in Stock or mild performance they will foul spark plugs all the time, especially if the fuel air mixture is a bit rich.

You may have something so simple as a fouling spark plug, they can do funny things.
Put a brand new set of spark plugs in it, mopars were Built for Champion Spark Plugs. Lots of guys are having good luck with Autolites. Hopefully you are running the mopar brain box and ballast resistor ignition system. Simple is good.

Give that a try, it either gets better or gets worse and leads you to the answer of how to get things right. Keep working through it.

Process Of Elimination !