Tach reading high

Steve I am out of logical suggestions. I would "dewire" the ignition, temporarily, and "hot wire" it to try and isolate. So clear back to the basics

1..Double check the tach ground wire
2..Check with voltmeter to be sure tack power is solid
3...Unbolt, clean and re-ground ignition module
4...Pull ECU connector and "work" it in/ out several times to scrub terminals. This is difficult because of the center screw. Some of them will come out
5...Then, disconnect the power to the ECU/ ballast and wire only that to a method of hot wiring, AKA maybe a temp switch taped and hanging in the bay, and connected to the start relay, or maybe even a good battery charger clamp, etc, so you KNOW the ignition is getting power.

Do you know how to check the alternator for ripple? Connect your voltmeter when set for AC not DC, to the alternator output stud and ground. Run car at high idle with and without a load. (lights, heater) Read AC voltage which should be very VERY low, the lower the better, and post that here.