Here are some factory shoulder belt threads.
http://67-72chevytrucks.com/vboard/s...d.php?t=467785
http://67-72chevytrucks.com/vboard/s...d.php?t=339651
http://67-72chevytrucks.com/vboard/s...d.php?t=455323
http://67-72chevytrucks.com/vboard/s...d.php?t=523162
And,some pics of some I found in a 70 or 71 Blazer that measure the same as those in the trucks. All that was left of the rears is shown in the center of the picture.