Someone here may know what they're made of but it's really a question for the online Trans Am/Z28 clubs. Someone there will provide an answer.
As for polishing, assuming they're not plastic or carbon fiber or something

, metal polishes like Mothers, Eagle One, Wenol, P21S, and Happich will all do a great job. Use a soft cloth or try some #000 super fine steel wool. I had a Mustang LX 5.0 and used to polish the exhaust pipes with Mothers and steel wool and they looked awesome.