74 dart vibration problem.

The torque converter is definately moving in park with the engine running. It's bolted to the flex plate which is bolted to the crank. The stock 360 is externally balanced which means it uses external means to balance the engine. In this case a balancer with an oblong weight ('71-7) or hollow slot cut into the outer ring ('78-up) and a weight or two welded to the torque converter. A vibration sitting still in park and revving the engine indicates it's somewhere in the rotating assemby from the torque converter forward. This is assuming, of course, that the mounts are in good shape and nothing is hitting the frame and the exhaust is not rigidly mounted. Hopefully, whoever built the engine also balanced it or you're in for a difficult search.