I found the one for my '81 G-10 van inside the front portion of the unibody frame rail one day,while doing an oil change,back around 2000!..I saw what looked like paper rolled up in a tube sticking out of a hole in it ,at first I thought (and prayed) it was MONEY someone stashed away in there,but after about 20 minutes of trying to carefully extract it in one peice ,I gave up,but was able to read enough of it to see it was the original "build sheet"...surprised it survived all those years and was still able to read it..much of it crumbled as I attempted to remove it though,unfortunately..(dont really matter,as the tranny is the only original drivetrain part left in it anyway!)..
I have found build sheets in pickups in between the seat springs,glued to the back wall of the cab,and on one truck it was on the top of the gas tank...my friend had an '82 C-10 that was a phone company truck--originally came with a 250 six,and a five speed manual,that looked like the ones GM used in Chevy Monza's!...a shortbed fleetside too...!