most Plymouths did not have the harmonic balancer however a mechanic I knew had worked in dealerships when these cars were new told me that if a customer complained about an engine which was not really smooth, the installation of a balancer from a Dodge would often correct the problem, Other sources of vibration should be looked for, like rear mounts that are too firm, a pressure plate which is not well balanced, fan blades and so on.
on my own car, bad ignition wires of the carbon fiber type set up what seemed to be a vibration on an engine which I had balanced/