As Jeff says that we should teach...
For 1967 mustangs to have dual exhaust from the factory, the rear floor pans (the area under the rear seat and going up on the rear axle tunnel) had re-enforcing sheet metal spot welded from the interior. I don't have a vast photo gallery of Mustangs as Jeff does, so I will rely on him to post pictures of the floor pan re-enforcement. I believe these re-enforcement plates came into use on all dual exhaust mustangs from the very beginning of mustang production. If your mustang does not have the floor pan re-enforcement plates then I doubt your 67 mustang came with dual exhaust from the factory.
Also the rear frame rail holes where the tailpipe hangers mounted typically had inserts. Having these inserts did not guarantee that the mustang came with dual exhaust as there are a few documented cases of the inserts on non-dual exhaust mustangs.
Through research and conversation with Kevin Marti, the two of us reached the conclusion that only S- and K-code mustangs received dual exhaust from the factory for the 1967 model production. All 1967 A-codes, regardless if it was a GT or not did not receive dual exhaust from the factory. Moreover, there never was an option on the factory order forms to specify dual exhaust either!
One point that Jeff made that I want to further clarify. The rear valance with the dual exhaust cutouts was only used on mustangs with the GT option AND either K- or S-code motors. While all K-code 67s came with the GT option, not all S-code 67s did. Thus, there were many S-code 67s with the standard the rear valance and turn-down exhaust.
Hope this helps!
Regards,
Pete Morgan