You can also buy a 12v to 6v electronic voltage reducer from Speedway Motors on-line, for your fuel gauge. It is called a Runtz, and is about $19. I used one on my '50 Plymouth, and it works great.
Aren't the temp and oil gauges mechanical? I'm not familiar with your gauges, but I thought they would have a capilary tube and bulb sender on the temp gauge and pressure tube on the oil gauge that runs through the firewall to the engine... Might have a look at the backside of these gauges and see if the tubes are still there, or if they have been cut off. If they are mechanical and the tubes have been cut, the temp gauge will have to be replaced because the bulb has some sort of gas inside that expands with temperature. You can probably find a replacement tube for the oil gauge, it is a simple tube that connects to the oil galley on the block.
The amp gauge won't care if it is 6v or 12v. My Plymouth was 6v positive ground, and is now 12v negative ground. I just reversed the wires on the back of the gauge to correspond with the change in polarity. Been working like new for years like this.
Pete
