1974 Dodge Dart Magnum Swap

I see, it was your 408 stroker that had a vibration. That is a whole new rotating assembly and requires its own balance be it internal or external. No parts off the original 360 magnum are going to work with that, they are meant to balance the weight of the 360 rotating assembly only. Maybe the converter guy is confused with the way the factory changed how they balanced 360 magnums, up to 95 the flex plate was neutral and the converter was weighted, 96 and newer the flex plate was weighted the converter was neutral, but the engine itself was always externally balanced.