To clarify for us "less than Ulf" readers, was the "Formula S" the same option as "Commando" and "HiPo"? Was it a term only used for Barracudas? It sounds like the dual exhaust was only for the rare Big Block engines (race only?). Those would have had a cut-out on the passenger side of the transmission support, which people do today to fit dual exhaust (TTI can mod. yours).
The AllPar site has a photo of a car showing the unique recess in the rear panel for the rectangular resonator. Besides the fender tag, that is about the only proof a car shipped with the Commando engine. I post a drawing from a FABO post showing the Commando exhaust discussed here. That Y-pipe looks a little hillbilly. Hard to believe a factory design, but I guess for low volume it made sense. My 2bbl Y-pipe is more elegant butt-welded pipes, but that required tooling. Real interesting that a single pipe made more power. I guess I won't be cutting my cross-member.