The circuitry is in line or series with the ignition. Install from beneath the dash and fire the engine up and see what happens. If the original wire ends are in place, there is only one way to hook up. If the tach is bad, there is a good chance the engine won't start.
On the bezel ring, you will need to find a none OEM Faria tach with a nice bezel to salvage. The salvage tach should be from the same time period.
Best of luck
Cory