Quote from s2msIMO it makes more sense for a (presumed) shortage like this to have more of an effect if it was at San Jose rather that if it was a Shelby specific modification. Plus Bo mentioned he has owned Export Mustangs with the reinforced towers.
Guess more digging is needed on this one.
We would expect that if they were installed at San Jose they would have been mentioned in the Add-Delete sheets that Ford used to build the cars. From what I recall they are not listed there.
As for export cars there may have been running changes during the years or have to do with the country the car was being shipped to, as the original ones I've seen and friends have owned did not have them as mentioned earlier.
Don't remember seeing them ( been a fair number of years though) any on Mustangs in Germany and Switzerland in the 70's. Most recent one I ran across was a NJ car (kind of odd) built for Japan. Most Pacific Rim exports I've seen were built at SJ but it may have more to do from where/how it was ordered