The 1984 to 1989 300zx has a very good gauge package from the factory, even so, there is room for improvement. The following will give you the installation points to acquire the needed signals and adapters to run after market gauges.

Gauge type	Signal location/ Adapter needed	Image
Air/Fuel Ratio Gauge	Locate pin #24 on the ECU. It is a solid black wire. You can also connect to the same Black wire off the stock o2 sensor though it will need to be run into the driver compartment.
Oil Pressure Gauge	You will need to get a 1/8 NPT to 1/8 BSPT metric adapter to install this gauge in your Z. It is Autometer part # AUM2269
Tachometer	Locate the blue signal wire off the coil and tap int this.