My first car was a '68. It had the shoulder belt in it (It was not functional in the least). The strange thing about my car is that it had '67 rear quarters. It was an early '68 model. A couple of people told me that Ford probably just used "leftovers" on it.
Interesting. I've been thinking about putting shoulder belts in my 68 and I wondered how they mounted. Unfortunately the headliner is about the only thing in great condition in my interior, so I don't see it coming out.