Richard,
While all of the pictures and details in this thread are great, keep in mind that in 67 only S-codes and K-codes could have come with dual exhaust from the factory (your signature cars do not say either). Sure a dealer could have added it, bit as Jeff has pointed out in the past, the dealership would not have done it to factory standards. Rather they would have taken the Mustang to their muffler shop and put on the cheapest they could.
Regards,
Pete Morgan