Habanero52, (love the "name" BTW) I've got a 70 350 GTO and have been searching here and elsewhere for that very same answer. To date, I've never found the answer.
I bought a NOS side cover from Richard for the sole purpose of having something original and unfaded that can be color matched. Once I get that done, I'll be posting the paint formula here for everyone to use, but that'll be sometime from now and won't help anyone needing answers soon.