I might be wrong, but I suspect seat belts were optional until they became standard in the early '60s. After the big safety buzz that Ford started in '56, a manufacturer would have had to be a little dense to not have padded dashes and seat belts as optional equipment.
Key thing would be to determine if there were seat belt anchors in the floorpan. Basically, that might be a reinforced area or segment of an area behind the seats, near the outer rocker panel area and on each side of the driveshaft tunnel. Otherwise, you'd need someone to make a reinforcment piece with an appropriate size nut welded in the middle of it, for the seat belt bolt to screw into from the top side and get that tack welded to the underside of the floorpan.
I kind of suspect that many of the earlier seat belts would be similar to what the add-on units or "replacement" lap belts of today might be. The thicker webbing came in more toward the 1964 or 1965 time frame, again, if I remember correctly.
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