1970 Dart Filler Neck Replacement/Restoration? Need Help

I wouldn't coat or paint it with anything- it's going to be constantly exposed to fuel, any degradation of a coating would cause as many issues as the rust would have. I leave them bare- as long as it's being used regularly, there should be no appreciable corrosion. Inside the tank and filler, it's all fuel and fuel vapor; minimal airborne water vapor to rust things up. Issues happen when the tank is empty for overly long periods of time and becomes exposed to moisture.