Installed new coil, now the tach acts funny

You need to make certain that "whatever color" that the power wire from the ballast goes to coil +
And that "whatever color" the coil - goes to the ECU

Something you might try "shot in the dark"

Unplug the regulator and see if the tach works

With everything connected normally

With the engine running "fast idle" Check charging voltage at the battery. With the engine warm and the battery "normalized" you should see 13.8--14.2

If so turn on the headlights. Heater if you have it. Voltage should not drop much, not below 13.2--13.5

Now, turn your meter to AC volts. Measure right at the alternator output stud with engine still running on fast idle. You should have a very very low reading, almost zero. I would say over .3V (3/10 of 1 volt) is too much. This is AC ripple caused by bad diode(s) etc.

If you have "anything" for a radio supression cap, install at the coil+ to ground.