Thanks for some real help. The situation with current actuated tachs is that they depend on the current waveform through the primary lead to the coil. They have few active components in their circuitry, they are basically current transformers with an RC/zener circuit to clip and modify the output into an analog voltage that feeds the meter movement. I know what the waveform for a points ignition looks like but I cannot find what the waveform is for the supply to a Mopar electronic ignition. And I haven't found it after hours of GOOGLING it. The HEI adapter seems to be the best bet at this point.