The original gas cap from '71 Dart GT was long gone before I got the car. The previous owner put a chrome '72 & up gas cap on it. It was "clocked" at an odd angle and was not flush. I gave up on the idea of locating a painted '71 gas cap and went with a '69 Charger flip top (with the adapter from FABO member (search for him)).
Another option would be to buy a used gas fill tube/neck from a '72 & up Dart/Scamp and replace yours. Here is link to one for sale currently:
[FOR SALE] - Filler neck Dart/Scamp
Good luck!