The 64 sedan had 3 point lap belts and a roll cage reinforcing the passenger compartment. Not really a particularly compelling example of a typical ‘64 sedan…
Neither my 1965 coupe or 1966 convertible Mustangs came from the factory with lap belts. Both have subsequently been modified to have (2 point) lap belts, which still feel sketchy as hell to me.
Have you used aftermarket 3 point belt? A family member's Ranchero had them; the shoulder restraint slid into the lap belt near the buckle, but if you moved it would slip out. I can't imagine that being helpful in a crash, just a tethered weight to fly around.
I fitted the lap belts to my 'vert, which was already very much not-stock when I bought it, so I didn't feel bad making even more modifications (hydraulic clutch, modern T5Z trans, electronic ignition). There isn't a good, above-the-shoulder mounting point for the shoulder belt, unless you weld in a cage or mount one to an aftermarket seat.
The coupe is a close to correct C-code, which I've driven under 1000 miles in the last decade, so I've decided to leave it with period-correct 1965 belts that a prior owner installed rather than modify the B-pillar as my exposure isn't very high on that car due to very low annual miles.