You have looked at the Paintref.com site?... it is listed there as PPG colour 60517....
Also there is a history pdf of an ebay auction for an Autumn Bronze car where owner says had to get PPG to talk to GM Historical to come up with a mix (17 pigments)... (search google for "Pontiac Firebird rare Autumn Bronze pdf", the April 10 2010 link has the info)... so PPG must have it.
Does anyone have any recommendations around the Autumn Bronze for 1968 Firebirds? Most "recommended" seem to be too bright... Appreciate any experience.