Curt,
I don't think you are going to find "factory" references as seatbelts did not really become a fixture in cars or trucks until the late 50's and then they were optional for a few years before really taking off. I think your best bet it to see what others have done here as street rod applications have been retrofit and available for quite a few years now. Here is a shot of mine I did three sets and used the ones offered from ClassicParts.
http://www.flickr.com/photos/advance-design_parts_co/8129063725/in/set-72157631867396207