I just pulled my FC out from sitting for 4 years. At first the tach didn't work but now it cuts in and out and when it works the buzzer sounds. It registers the right RPM though so I know it's getting signal. I don't know if it's related but when I have the headlights on the double beep alarm goes off like if I had the car off. Also my oil pressure gauge goes all the way up slowly as soon as I start the car. I'm sure if it was really that high there would be engine problems. Any advice would be appreciated.

 

Check the condition of the trailing coil connector.

Also, make sure all your grounds are in good shape. The major grounds are listed here:
http://www.aaroncake.net/rx-7/grounding.htm

 

I got a used dash with cluster and gauges. nice condition. realized it was for a turbo because of the boost gauge instead of a battery gauge. will it still work? can I make it work?

 

I think you can just swap the boost gauge with the voltmeter.