Depends how the aftermarket gauges have been "adapted" to work. The original oil pressure gauge is mechanical - is the aftermarket one mechanical or electric? If both are functional, is there a tee somewhere in the line?
The aftermarket temp gauge probably has its own sender - is it mechanical or electric? Where is the sender for the aftermarket gauge? Is the original GM sending unit still there somewhere?
The aftermarket voltmeter connection is simple - they're just spliced into any ignition-switched 12V wire. The original ammeter is completely different, and is in series with the car's main power feed circuit and its loads; has the original ammeter wiring been "messed with"?
You never know what a previous owner has done or how he did it until you poke around and see exactly what was done.