Was for sale in August by same seller.
There are several items that's not original, -not just what could be the fuel tank..
The fuel tanks were galvanised from factory. Whether that's the original tank or a replacement, it does look like it could be a GM. Even a mild rear-end collision would damage the tank, so a replacement within the last 42 years is likely. Many non-GM repros look like, or are painted.
This car has an engine number T1230HC and I'm wondering why the seller has failed to mention it. Car's build date is 2nd week of Feb.
The HC is 350/250hp (RPO L65) 4 speed (could also be 3 speed manual) and 2bbl carb.
http://www.camaros.org/lm1.shtml#L65