tach issues

I would also check ground connection, from engine to chassis. Is tach wiring grounded at engine or chassis? How and where is the HEI grounded, and tach wiring grounded? The distributor body needs a ground too.

Tachs are triggered by the (-) coil terminal going low to high. If grounds are poor, there will be voltage such that the signal does not go low enough. It must also go high enough. It goes high, a few hundred volts when ignition happens. At higher RPM, the ignition interval is cut short, to alow more time to charge the coil. It could be the tach has a problem with that.

Using a high voltage probe and scope on coil (-) may be helpful to visualize the voltage.