Upgrades on gauges questions

The 2 wires on the amp gauge are hot at all times. Current passes straight through this type gauge. You'll need to connect those 2 wires together to keep everything powered. A volts gauge is different. It gets a switched hot and a ground. There is a switched hot wire to your OEM inst' panel. It will be just black or blue with white tracer. It fed switched 12 volts into your inst' voltage regulator which was probably inside your fuel gauge ( its a 3 post gauge? ). Factory wiring diagrams would help you.
The OEM thermal gauges and senders operate on 80-10 scale so that's the type gauge you'll need when using original sender. Aftermarket fuel gauge will not have a voltage limiter/regulator inside. Aftermarket gauges may operate on 12 volts and not need a limiter/regulator.
How you deal with illumination, need the circuit boards or not?, I don't know.