I have boxes of seat belts, and probably 30 various 65 66 black, all flavors.
That being said, the seat belt material back then was rated for 18 years. It is way past safe any more.
You can pull it as hard as you want, and it will hold, but may not hold in a 30 mph accident, so . . . .
It the belt for you, or for someone else?
I do not rebuild seat belts, but know people who do, to current safety standards. I will be glad to sell you a core belt for rebuilding, just with the webbing slashed. I do not want to be responsible for death / Injury due to a 50 year old belt failing.
ed ay +3112
1968 GT500 Arrest Me Red (The Big Boy Toy)
1966 K Code Vert (The K 'Vert)