I put a Pertronix ignition module into a 1971 Pantera recently. It had breaker points prior to the conversion. The conversion required a power source for the ignition module that routed around the ballast resistor. The tach worked normally after the conversion and the car ran better.
I think the tach adapter is needed when you go the MSD route. And the tach adapter is a bit of weak link. It can be damaged when charging your battery. If your battery is in doubt it may be better to replace it than to charge the heck out of it and damage the tach adapter in the process.
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