tac problem
The tachometer in your 74 isn't electric, it's driven by a cable that runs from the base of the distributor through the firewall to a connection at the rear of the tachometer body.
The fact that it stays at 2800 rpm indicates at least part of the problem may be in the tachometer mechanism itself.
Getting at the back of the tach isn't easy…. requires at least loosening the left side dash pad. Are you prepared to do that at this time?
You might also try disconnecting the cable at the distributor and using an electric drill to spin the cable to see if that has any effect on the tachometer.
Do you have a 74 Assembly Instruction Manual? That will help you see how the cable is run.
Sorry there's not an easy solution.
Regards,
Alan
The connection on the back of the tachometer.

The connection at the left side base of the distributor.

Mine is a 75 with electronic tac,,also my mistake its at 1800rpm









