Though our trucks are equipped with threaded holes on the rear door pillar area for attaching the 3-point seat belts, and they WERE an option, they're very rare. There are aftermarket companies like Juliano's Hot Rod Parts who can supply you with something very similar, but oftentimes people just would rather use inexpensive readily-available factory parts. Since the next generation of Ford trucks did incorporate shoulder belts, as such:
...many people have tried to use these in the '67-'72 range, but they don't always look the best, since they can't be frenched into the rear door pillar area like the factory did. They end up looking like this:
...where they protrude, are unsightly and are where you can keep hitting your head on them. However, there an auction on E-bay right now for a '67 F100 with a slightly new twist on this dilemma...he's actually frenched the shoulder-belt apparatus INTO the door pillar! Very cool! I'm not sure how this was accomplished, but it looks very good! I just wonder why he didn't opt for also using the factory plastic covers:
