[url=https://www.gmheritagecenter.com/docs/gm-heritage-archive/vehicle-information-kits/Chevrolet-Trucks/1963-Chevrolet-Truck.pdf]I do not see seat belts as offered as a Factory Regular Production Option in 1963.[/url
I'll be wrong, again, if it was an option in 1963/64.
1962 - factory RPO in Chevrolet cars
1963 - several states started requiring front seat belts
1968 - Federal Mandate