Lap belts didn’t become standard equipment until–I think–'64 or '65. Our '63 Plymouth didn’t have seatbelts, but Chevron was running a nationwide promotion featuring front lap belt installation for a really low price, so my father took advantage of that.
Our '66 Galaxie 500 came equipped with lap belts, but I think that they were only in the front. The Federal law mandating lap belts for both front and rear didn’t take effect until 1968.
My '71 Charger was the first vehicle I drove that had shoulder harnesses in the front, and the design was really bad. They didn’t retract, and if you didn’t want them dangling in the door opening, you had to fold them and stow them above the window with clips that were provided. It was such a PIA to stow and to un-stow them that I rarely used them, and instead relied on the separate lap belts.
The biggest problem with the Charger’s shoulder harnesses was that they didn’t have an inertia reel mechanism, and if you leaned forward they didn’t yield. As a result, it was impossible (at least for me…) to reach the e-brake release when I had the shoulder harness attached, so needless to say, I rarely used them.