How about an update. This is an important question to me.
As a kid, I only wore lap belts, until the day I buried my then 71 Duster under a semi-trailer. I will save you the medical ramifications of doing this.
Doctors told me that if I had been wearing the shoulder belt, I would have still been badly hurt, but nothing like I was.
So since that day, I have always wore the shoulder and lap belts. To this day I will not ride in a car without buckling the car into me.
I want to convert the original belts to the 3 point system of my new 71 duster