This ATL fuel cell ad from back in the day says the tanks were "metal". I feel pretty confident that back in that
1972 and earlier era, the fuel cells were painted steel, not aluminum or stainless. Prior to the fuel cells becoming
mandatory in April '68, many cars were running the factory steel gas tanks and often it was two factory steel gas
tanks so they had extra capacity. In April '68, maximum gas capacity was set at 22 gallons.