It is not based on the plant, Chrysler keeps records of all build cards for cars up to 1967. I don't recall why the records end there.
My buildsheet was an animal's nest, so getting the card was assume. I also learned a few things and was able to follow up with Chrysler via e-mail and obtain more information.
The thing that was most interesting to me was the fact that the date on the fender tag was not the actual production or shipping date for the car- it was a scheduled shipping date. The build card list the scheduled build date and the actual shipping date, which was 5 days earlier.