I installed 3 point seat belts in my '65 SF. There was a hole in the roof structure in the exact spot it needed to be. There was no doubt in my mind when I saw it that the factory already had the shoulder belt set-up in mind for the all new c-body for '65, just didn't add it. Instead of just adding a nut, I cut the rocker panel nut section from another car because I wanted the heavy duty nut with a seat belt bolt. It was about 5" x 2 1/2". I enlarged the factory hole for the large nut on the back of the patch, placed the patch/nut on top of it and welded it into the roof. I ordered seat belts for a '70 Road Runner. I like how the shoulder belt can stay up on the headliner when I don't want them hanging down. I don't have my interior in yet, but I've had the seats and seat belts bolted in to test fit everything and it's really nice, fits well, and just feels right.
I ordered (have received) a complete interior from Pro Fit Interiors. I also ordered my seat belts from them, all 5 of them - I added 3 for the back seat. Might be worth checking them out.
I have a nice set of red factory front seat belts for a '65 SF if anyone needs a set.