Can't say for sure on Chevelles but as far as I know the chambered exhaust was not standard but an RPO option that had to be chosen. It was canceled for production as of May 19th, 1969 for Camaros and I would assume the Chevelle as well but it does not state it in this bulletin. Link to the cancellation bulletin for the Camaro is below, I also have several other versions including a letter stating why it was canceled due to noise but have yet to scan them.
1969 Camaro Chambered Exhuast Cancellation Bulletin