All I remember about my 69 Sportsroof (traded in for a 73 Torino) lap belts were that they came with a "stretchable" elastic strap. Back in those days, you seldom, if ever, got a ticket for not using a lap belt, so they were left in the stowed position. The strap needed to be wrapped several times around the roof post to keep it from sagging too low.
Jim