Ditto on factory tach being off. Also, I tried a tach adapter and that too was unreliable.

I have had three rallye tachs upgraded to accomodate for a multple spark ignition system.
If you go this route, depending on who you use, you likely need to specify that you will be using a MSD type ignition. Otherwise, they may rebuild/recalibrate for a factory system.

I tried the tach adapter however, my tach was off so I didn't trust the information that