My 70 351C cougar eliminator has them.
I am a Cougar guy so maybe the Mustang being cheaper why bother LOL
This is all well known information and can be traced back to books used to judge these cars in concours classes. Just because you have not seen one, does not mean they do not exist. I have seen plenty, so perhaps you need to see more cars.
Again Ford was not consistent on this and depended on what was on hand is the common belief. The 351 seen them both ways. Have been told depended on options like of it had the comp suspension, A/C etc... but honestly never looked into it that far. If you are really interested contact the guys that do concours judging. Btw some cars also came with stock type towers that were welded. That supposed to have started around Dec 67, and the re-inforced wrap around towers around April 68. Been around Cougars over 40 years, and used to judge some concours classes back in the 80-90's so what I know is from those guys who honestly are way too nuts about this original stuff and why I stopped doing it.
Heo, the 2nd torque box started for the 1968 model year, at least for the Cougar which was only a coupe in 67/68. The one piece shock tower brace was used on shelby and cars to be exported. Guess why a lot of guys call it the export brace.,