Most if not every steel gas tank from the muscle car era was made of ASTM A308 terne sheet, which is steel sheet coated with an alloy of lead-tin by the hot dip process. See http://www.astm.org/Standards/A308.htm

This spec discusses the coated steel sheet is suitable for solderability, corrosion resistance, and gasoline resistantance.

As far as I know, all reproduction gas tanks are made by Quanta. See
www.gastanks.com

Quanta states that all their tanks are made of “Corrosion Resistent Ni-Terne steel”, which sounds close to A308 or the same as A308, except A308 does not mention Ni (nickel) in its title. (You can’t read the details of A308 chemistry online unless you pay money.)

New Quanta tanks are shiny, and old NOS ones would have some oxide of lead-tin on the surface. I’m not sure what color tin turns after long atmospheric exposure, but I think lead turns dark gray. I suspect the only difference between new and old terne sheet is just discoloration from atmospheric oxidation, as long as there weren’t other sources of corrosion.

I suppose it is possible there is someone out there making tanks from galvanized sheet. It's really hard to tell the difference between terne and galvanized sheet just by looking. It should be better to solder the two halves together and make a leak proof joint with terne than galvanized (zinc).