1....Might be the tach unit is bad regardless of who "rebuilt" it
2....Might be a wiring error or a short in the tach wiring.
3....What do you have for ignition system?
If you have MSD CDI (or any CDI) ignition, you don't hook the tach to the coil.
I would start by pulling the tach wire loose up in the cluster. If the engine will then run (watch the wire it will be "hot) then it's wired wrong at the tach or a problem in the tach.