I'm  wondering about the earlier years when they first started being an option. ...1962,1963 or 1964

It may depend on the body style as well as the year. Sport Coupes & convertibles I believe had belts for 4-positions. It didn't seem to matter if it was a big car or not.

My '67 Chevelle sport coupe (o.k., it's an intermediate) has belts for 4 people but was officially a 5-passenger body style (sedans were 6-passenger but still had belts for 4). In the U.S. you could optionally order center belts for the rear position, but not in Canada.

I don't think belts in all positions were standard until 1968 or 1969. For 1968 shoulder belts for the front outboard passengers were made standard on all except convertibles.

 

I can't be certain how many belts were in dad's '66 Biscayne as nobody used them back them & they disappeared behind the crack in the seat cushion. Besides, dad traded it in 50 years ago. I think there were only 2 rear belts though (this was 1966 after all). I remember back in the day when dad had to hit the brakes, his arm came over to hold us on the front seat. How cute and utterly useless.

2 front and 2 rear. My Dad's 1963 Wildcat had 2 front; 2 rear. I bought a 1966 Laurentian from the original owner with 30,821 miles on it. It also had 2 front belts and 2 factory rear belts. He added an aftermarket one to the rear, middle.