Found something similar a couple years back and want to share what worked, because I almost gave up twice. My grandfather's '66 GTO got sold in the 80s and we figured any paperwork died with it. Turns out my aunt had stuffed the original sticker inside the owner's manual, which sat in a box with old tax records for forty years. So before anyone writes off a paper trail, hit every relative's attic, every old filing box, and every bank safe deposit box your folks ever rented. Dealership files from that era are long gone, but family hoarding habits are forever.
If you do turn one up, handle it gently. Keep it flat, out of sunlight, and slide it into an acid-free sleeve rather than laminating it. Lamination kills the value and can't be undone. Take high-res scans both sides and store copies separately from the original. For verification, the build sheet numbers and the VIN on the sticker need to line up with the car's data plate. Pontiac historians on the GTO forums can confirm a geniune label fast, and they love doing it. If the car itself is long gone, the document still has real value to collectors and to family history, so don't toss it just because the goat is gone.