I found mine under the rear seat, I got very lucky as mine was readable and in good shape, I scanned it into my computer so that I could refer to it and not damage the original. THE HOLY GRAIL has been found!!!
I got my car new, so I was lucky. It is a little known fact that Chevy shipped the cars with a copy of the build sheet in the glovebox. I ordered my car and picked it up before my dealer had enough time to "clean it up" and dispose of that copy. The "glovebox copy" is special because it has inspection stamps on it for the final line inspection of the car.
Currently I have the "glovebox copy", another copy from the gas tank, and a third copy that was under the front driver's seat.
If you find any copy, treasure it because build sheets are reliable documentation of the car.