I believe they were originally zinc dichromate plated which is also what was used on Holley carb bodies. Best thing to do would be to check with a carb shop, maybe Woodruff from this site or All American Carburetor in Florida and ask what they would charge to dip them for you. Otherwise google something like zinc dichromate spray paint and you'll come up with some ideas on how to duplicate the finish.
Mine were replated zinc dichromate and I've always thought they came out way too yellow. Survivor cars seem like they are closer to a grey color like the ones John shows.