I suspect your fuel gauge circuit is not making its way back through ground. We depend on fifty year-old rusty spot welds to bring the ground back to your battery.
Modern cars use a separate ground wire right in the plug at the tank AND they ground the battery to the body right at the battery. You can simulate some of this on your car:
Ground your battery to the body using a short #10AWG copper stranded wire.
Drill a hole in the corner of your fuel tank and connect a #14AWG stranded copper wire from the fuel tank to the frame using brass or stainless screws and nuts. Be sure your connections are tight. Do it like this:
Notice that the hole is away from the seam weld, anywhere on the outside of the flange.
This should ensure gauge accuracy. Also, pull the sending unit out and physically move the bulb. Some of these have become filled with gasoline over time, also giving false readings on the gauge.
The "at rest" position of your gauges with the key off for some time, should be, "empty" and "cold". You cannot connect gauges the wrong way meaning, they are NOT polarity sensitive. The movements are activated by current which produces heat on a bi-metal strip which deflects the needles. Also, you MUST have a solid ground on your CVR or they will deliver steady 12-volts to your 6-volt gauges. - Dave