After Market Fuel Gauge 1966 Dart?

You should be able to use one of the Auto Meter fuel level gauges that are calibrated to 73 ohms @ empty and 10 ohms @ full. They should work with your current tank sending unit and be pretty close to the factory gauge. Make sure that your tank sending unit has a good ground.

I ran a ground from the fuel tube on the sending unit to a ground on the unibody. I will check it. Did you splice the Auto Meter in and left the old one place, or just by passed the original? Thanks