is it possible to put a shoulder/lap seat belt in my duster?

My 68 Barracuda has the mount on the roof for shoulder belt to attach to so I would expect your Duster to have them too.

JC Whitney had universal shoulder belt kits for a resonable price. These were a more modern design where the retractor for the belt is on the floor and just loops through mount at the roof than the seperate belt that needed to be adjusted for the shoulder belt.

I plan to add them to my Barracuda this winter.