I had that problem with my 74. After taking the seat out, etc, I found that the retractor was water soaked and full of corrosion. I was able to take it apart, clean, polish lubricate, and reinstall the unit. It seemed to work fine after that.
Thank God I did not have to ever find out if they worked in the intended use however. There was doubt in my mind if they had some much trouble going in and out whether or not they would properly lock in a collision.
Go throught he Portal at the top of the screen and do a search. You will find several options with vendors on seat belts.