You'll are making me work too hard on a Saturday morning lol. Follow the link here to the '68 Mustang new car brochure. Look to the right side under standard safety features and part way down the list. Note the quote "Here are some of the many safety features you will find in EVERY Ford built car in 1968". This same list is in every '68 Ford, Mercury & Lincoln brochure and similar lists in following years.
http://www.oldcarbrochures.org/NA/Ford_-Mustang/1968_Ford_Mustang/1968_Ford_Mustang_Brochure/1968-Mustang-15My mom bought a new 1968 Cougar Feb 29th, 1968, I rode home in that car from the dealer and I still have it, it has shoulder belts. I also have the original sales receipt too with the options, no charge for shoulder belts. My dad used to get a new company car every year back then ( sadly almost always the cheapest full size Scrub ) And I remember that car had shoulder belts too. I was a kid into cars and stuff like shoulder belts, side marker lights, were big news back then. I've had 3 1968 Torino's ( still have one ), every one had shoulder belts. I had a '68 Galaxie once, it had shoulder belts. I can't answer for Canada, and I've certainly never seen every Mustang made, but I believe starting in 1968 they all came with shoulder belts JMO.
I will say as others have noted, they were generally a pain in the ass back then, often separate belts, chaffed your neck, etc. Rarely did anyone use them. They weren't retractable, they had an elastic strap that when the belts weren't in use was supposed to go around that clip on the headliner. After a couple years the elastic would stretch out and the belt would droop down in your way. They also had a rubber sheath the metal end was supposed to slip into, but again, after a couple years the rubber shrunk and you could never get the belt to tuck in there. It was not uncommon for people to remove the shoulder belts or just plain cut them off.
