I don't know how to help you but on my 79, the owner's manual still has the saleman's business card stapled to it showing where the truck was purchased. It ends up being just down the road from where I currently reside, however, the dealership is long gone.
I still have the window sticker and bill of sale and all paperwork on work ever done to my 65 Mustang GT convertible that i got from the original owner in the '80s.