IIRC, the Z usually signifies a service replacement for the fourth digit, not the part installed on the factory floor. If the Z is in the third position, it means that it was designed specifically for the Mustang, but could be used by other models. Most of the Mustang parts that are not sheet metal were designed by other lines, and not necessarily Mustang-specific. Radiators particularly fall into the latter category. The MPC (Master Parts Catalog) has a 67 Mustang 390 radiator as C7ZZ-8005-B, but all that shows is the service replacement.
My other source of documentation also suggests a C7ZZ part number for all of 67.
For what it is worth...