I just checked the Rocky Rotella "Definitive Firebird and Trans Am Guide 1967-1969" and found some interesting trivia about the subject. It says that it was planned that all coupes in '68 were to have shoulder belts but an internal document Aug. 23, 1967 rescinded the plan. They continued with them as optional. 26,616 coupes (about 30% of production) got the Deluxe Belts and Shoulder Harness option in 1968.

Edit 3/30/25: After reading the following post by Dennis, I realized I mis-read the option above. I likely is simply the upgrade to Deluxe belts (lap and shoulder) The 754 option adds front shoulder belts for both standard and Custom (deluxe). The Rotella book shows production numbers for convertibles but not coupes.