This is what I have found so far. Starting in Jan. 1968, cars were federally mandated to have anchorages points for shoulder harnesses. Here is a quote from Hagerty Insurance web site.
"President Lyndon B. Johnson’s Highway Safety Act of 1966 was a landmark piece of federal legislation that gave Washington sweeping powers over car design. The act specified mandatory safety standards for 1968 models. In addition, the government “convinced” Detroit to add six standard safety features to all 1966 domestic cars that included seat belts. Starting in January 1968, lap-belt anchorages for each front and rear seating position and upper torso-belt anchorages at each forward facing outboard seating position were required. On January 1, 1972 , the same requirements went into effect for U.S.-built trucks."
Here is a link to a US Transportation report published in 1972 about laws requiring seat belts. From the way I read it, on page 9 it says that in 1968 only the state of Nevada and maybe Virginia were the only states requiring harness belts be installed.
http://ntl.bts.gov/lib/25000/25300/25318/DOT-HS-820-226.pdf