I have a 1985 trans am with a 355 v8 engine in it. I have a stock 305 instrument panel in it. The tach reads around 2k at idle which it is really reading 750 and it burries the tack when I'm really running at about 1500. My fuses are good and I'm all hooked up correctly on the distributer

 

3rd gauges are known for not working right over time might need to swap out the gauge

 

If you pulled the motor and the work yourself then it is likely some of your insulation on your wires got cut. Check the entire harness in the engine bay for small cuts in the wire insulation in the the wires. I had the same problem and inspected the harness for cuts and shorts. Found a couple and tape'em up with electrical tape. Good as new. I just did another engine transplant and I got the same problem again. If it doesn't work then it could be a resistor in the cluster. That means time for a new one. Best of luck!