I just installed 3 point seat belts in my daughters 67. if you have the headliner out or are willing to take it out ggregory has it right. however if you don't there is another option that is ok but probably not quite as good. it mounts the upper post behind the seat and through the door jamb.
here is a link to where I got mine. Mustangs Unlimited - The Premier Source for Mustang, Shelby and Cougar Parts and Accessories
I used seats out of a 94 mustang that have the shoulder belt guide to keep the belt in the right place as well.
if you look closely at the attached picture you can see the cad plated mounting hardware in the door jamb. then you have to drill a hole in the floor to mount the retractor. the other two mounting points re-use the stock holes.