If you want a silver finish, Rodger, it is probably best to have a clean and grit blast, then wire feed metal spray coating of aluminium, protected from dirt and staining by a silver UHT paint on top. Paint may or may last without the aluminium; and aluminium has been used for that sort of high temperature corrosion protection from as long ago as 150 years, I understand. The fire grates back then were not metal sprayed in the same way we use now, of course. I would seek someone who has Metco Thermospray or electric arc spray equipment, and get them do do a small piece of scrap first so you can see you are happy with what you get. Cadillac mostly used a nice black stove enamel gloss finish on the 1920's V8's that I have and am most familiar with, and I thought that carried on with the 12's and 16's. It is probably not beyond the technical capability of some restorers to re-do that at home either, if you had a manifold perfectly clean and improvised a gas furnace with fire bricks and blanket, and could be sure you got no distortion.
Ivan Saxton