I believe Snake-Oyl products makes repro seat belts, and the day will come when I buy new ones for my car. The originals are fine except they're very hard to adjust. But the underlying question is, did Ford offer rear belts in 1964? I would think so, but don't know. Front belts were standard, and rear ones were most likely an option. I could care less about rear belts, as nobody ever sits back there, and it would be a rare occasion for it to ever happen.
I don't know how the laws are in your state. In Florida, all FRONT seat occupants need to wear belts, and anyone under 18 needs to wear a belt wherever he/she sits. You're exempt if you drive an older car that doesn't have them.
With that in mind, I have a 4-year-old granddaughter. She needs a car seat, and there's no way to install one in this car. So I don't know how the law works in that regard. When she's a little older she might have to sit in the front seat because the car has no rear belts. Again, not sure how the law works. But she lives 500 miles away, and has never ridden in this car anyway. If this were to happen on a regular basis, naturally I'd be getting rear belts, and possibly a seat that would work with them. But for that possible once-a-year ride, I'll comply with whatever the law says but physically I couldn't care less. Remember we did fine without them in 1964 and before.