this is from the washington state patrols questions and answers on their web site. "I have an older vehicle. Is it required to have a seat belt?"
Every vehicle manufactured or assembled after January 1, 1965, shall be equipped with at least two lap-type safety belt assemblies for use in the front seating positions.
Every passenger car manufactured or assembled after January 1, 1968, shall be equipped with a lap-type safety belt assembly for each permanent passenger seating position.
Every passenger car manufactured or assembled after January 1, 1968, shall be equipped with at least two shoulder harness-type safety belt assemblies for use in the front seating positions.
Every pickup manufactured or assembled after January 1, 1972, shall be equipped with safety belts.
Rule of thumb: If your vehicle was originally equipped with safety belts, they must be in good working order and must be used.
so the old trucks are grandfathered and it's your call . (in WA that is )

Brice