Years ago, there used to be a office at GM where you could write and get a whole "restoration package" for your car that had all the info that they had. I've got one for my '65 Corvair. I doubt that they still do it now (for free at least). I did get a copy of the original invoice for my '94 Impala SS a couple of years ago.
The build sheet for my '79 Trans Am was behind the rear seat back.