The results are in. I rewired the power to the gauges so that the water temp,
oil temp, and fuel gauges were supplied by a stable 10 volts, using a DC voltage reducer from Power Stream. All other gauges are still supplied by the normal car voltage.
I put the dash back in and took a short drive to get everything up to temperature. Everything seems to work fine, but the fuel level is still on empty. There is between 3and 6 gallons in the car, so maybe there is not enough to register yet. I'll fill it up in the next day or so.
The voltmeter is also working now. I had the power and ground switched. When the car is running, it now shows 13 to 14 volts, depending on the engine RPM.
All is well. Thanks again for the help from everyone.