GM used in addition to the obvious Chromium plating, the metals Tin, Cadmium, Nickel, and Zinc for plating. GM used black oxide on all bare steel sheet metal after it was formed, which is cheaper than paint and does a better job of rust protecting the parts (though it fails as soon as the oxide is scratched off).
http://en.wikipedia.org/wiki/Black_oxide
I am sure I missed a few. You can buy every OEM bolt that was used by GM, with brand new coatings on them, from many vendors; to build a white glove concurs correct museum piece. I wouldn't try and replate them with an Eastwood kit or anything.
I do not like reusing fasteners anyway as they are only good for four to five torque applications before the threads start to deform. That is why in every engine I build I replace every nut, bolt or stud in the engine with new ones from ARP.
Big Dave