Yes, John (67gta289) is 100% correct on this being avaiklable for 67 Mustangs, as well as other Models. As a matter of fact, they were available from the beginning of the Model year, as evidenced by them being listed as an Option in the August,1966 Mustang Sales Brochure (copy attached). They also appear in the Ford Car Facts Book usually also printed in August of the Model Year ; my original includes Revisions from October of 1966), and in the October, 1966 issue of "Ford Parts and Service Merchandizing News", copies of each attached. Note that in the Ford Car Facts Book, this is shown as a Factory Option only, yet in the Dealer only Ford Parts and Service Merchandizing News, it is also noted as being available as a
Dealer Installed Accessory. Unlike most Accessories, this was not available to the General Public, most likely due to Safety/ Liability concerns that Ford had. It is not even listed in the November, 1966 or May, 1967 Ford Price Lists.
To complete this "exercise" on 1967 Mustang Shoulder Harness Seat Belts, I am attaching copies fromthe 1967 Mustang Bodt Assembly Manual. Note again, that these were only available for the Front Seats. Add all of this to what John previously Posted, asnd you will have the complete story, with documentation.
Hope this helps.
Bob