Help needed with 3 point seat belt!

If I was you, I would upgrade to some new 3 point belts. I can't imagine almost 40 year old belts are going to be all that reliable when you need them to be 100% reliable.

I can send you pictures of my new belt install on my 73 Swinger, if you like. I imagine the mounting location is the same as your 76 Duster.