Yep...really tough to establish beyond a doubt if this pertained to Firebirds at all. I have it that Firebirds were eventually produced in the St. Therese, Quebec plant up to 2002. The myth most likely doesn't pertain to FGFs but stands a good chance of pertaining to later generations.
Will keep diggin'!
Mark
68 Firebird 350 auto (sold) 70 Trans Am RAIII 400 4-speed (sold) 2011 Challenger SRT8 IE392 6-speed (sold) 2017 Challenger Hellcat 1966 Dodge Coronet 440