Ditto to all that was said, originals were stamped from galvanized sheet stock. If you ever have the chance to look at a original car, you might notice the edge of the dust shield may show some rust. Proof that they were stamped from galvanized sheet stock.
Maybe Post Sedan will take a look at his new survivor silver L78 Chevelle and have a look.
And yes, your only option now is to plate in white zink or cad. Thats what the repops are.
Cad is great if you can find a plater to do them, holds up much better than zink, IMO.
here is a shot of my last batch of Cad.
Mike