I just finished restoring a very late (July 15 built) 1971 Mach 1. The stripe is readily available from any of the big Mustang parts suppliers. When 71 and 72 Mach 1's were ordered with the Mach 1 side stripe (it was a separate option!), it omitted the thin bright trim which covered the paint break, and used the double (3/4" and 1/4") tape stripe to cover the paint break. Hope this helps!
Ken