Rear window thickness

[75gmc454]

I may be bringing up an old topic but I have not had any success finding any info in some of the searching that I have done.

A buddy of mine bought a 74 custom deluxe with a broken rear window. Well we went to a junkyard and found 2 rear glasses. Both were soft ray and both were out of 73-87's, but one was thicker than the other. I told him I thought the older trucks took the thicker glass, but he bought the thinner one. He put the glass in, but it does not seem to fit the rubber seal right. What glass is the truck supposed to have?