Shoulder belt adjustment

My 68 AMX was the first year of shoulder belts required later in the model year. My car has them and they are in good shape. But they flop around, even when held in the plastic clips behind the seats, and they are too long to use effectively. I am wondering if these had the old style spring retractors, like the lap belts? Even if they did not, it would not be hard to put them on and make them more useful to use.

Too long? they can be adjusted to become shorter

I have the opposite problem on 68 AMX - barely long enough.  I have them adjusted pretty much as short as they will go.  Length ranges from around 42" to around 52" from the far end of the plastic cover to the far end of the chrome buckle.  Pretty sure they are original to the car as all the belts have the "coarse weave."  I use them regularly so didn't bother to install the holders at the rear kick-panel.

Storing the belts is not an issue, but they are too long to wear even when fully shortened. They look like the original but most manufacturers used the same type of shoulder belt back in the day. I have ordered a pair of the old style retractors to tighten them it when I wear it. Shortening the belt by cutting and sewing is probably not a good idea.