Gas Cap

I did go to auto zone just for Shi@# and giggles and ask for a gas cap for a 71 duster and it came up N/A but then he ask if it was a valiant well i know that they use to be call plymouth duster Valiant, well he had and it fits real good BUT it's chrome, chome looks good on bumpers but not on gas caps,I guess that it will work for now so i can drive my car BUT here is the 100$ question what filler neck do i have need less to say i still need a gas cap!!!!