You'll have to look around and see how the AM gauges were installed. But I'll bet all they did was cut the leads to the variable resistance senders for the individual OEM gauges and tie in the AM gauges.
Sounds like you're describing the cockpit of a jet but there are actually only 2 gauges to deal with. Oil pressure and coolant temperature.
I doubt they installed an aftermarket gas gauge and the voltmeter has no variable resistance leg - it just senses system voltage via the GAGE/IDLE fuse.
The gas gauge is a separate issue.
On the voltmeter make sure the GAGE/IDLE fuse is installed and not blown:
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As far as the temp/press gauges: Best to start at the senders and trace the leads back until you find where they cut and spliced in for the AM gauges. When you find the cut/splice, reconnect as factory and see what you get.
The coolant temp sensing leg is a dark green wire that comes off the sender (in the head between 1 & 3 cylinders). It runs back along the valve cover and enters the harness near the distributor (drivers side).
The lead for the oil pressure gauge sensing leg is a single tan wire that connects to the bell shaped sender located behind the distributor passenger's side:
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