1987 2.8 firebird base has no tach or gauges for oil pressure or tempature. What is the best way to convert to gauges using my stock output wires at the cluster. Is aftermarket gauges my best option?

 

It's possible that the wires already exist in the interior of your car. If you pull your gauge pod out and look at the connectors behind it you will be able to see how many wires are actually populated in the connector. A GM service manual for your vehicle will show you the pinouts of which wires are which.

It's maybe less likely that the wires are already populated in the engine bay, but you can do something similar by inspecting the C100 connector for whether the wires are populated.

If nothing is populated then it might be easier to use aftermarket gauges and put wires thru the firewall somewhere.

 

I've done it. You need to re-pin the two connectors (C1 & C2) in the dash behind the instrument panel per the Factory Service Manual. There won't be a tach wire on the passenger compartment side of the firewall, so you will have to add that. The tach wire should already be present in the engine compartment, but may be black instead of white. You will need to buy the correct pin to connect the tach wire to the C100 connector terminal unless you want to run the wire through the firewall some other way (I didn't)...