Assuming your Chevelle was 25 years old when you first got it, is there the slightest possibility that somebody may have removed the original seat belts? And if there are no holes in the floor pan where the attachment bolts would be, maybe, just maybe a 25 year old car from the north east (rust belt) may have had new floor pans installed.
Google seat belt laws, that's where I got the info.
I'm not trying to start a war or anything, I was just pointing out that the auto industry offered seat belts before Ralph Nadar saved us from ourselves with his book.
Cgocifer, your car could have very easily slipped by the seat belt installer, quality control back in the 60's wasn't the greatest it could have been. My friends 65 Malibu had factory belts, they saved his butt when he totalled it in 1968 at 60 mph when a car pulled out in front of him.
Huudoo, sorry for the thread hijack. You're absolutely right, it's amazing how this car has been on the road "unchanged" for over 4.5 years and people still come up to me and ask "What is it?" (Of course, that sometimes happens when I'm in my AMX too.)