In most cases, the dealer would not install headers or cowl plenum air cleaner. Most dealerships would void the car warrantee if installed. There is no definitive answer. If you had an on-going relationship with a dealer, a very small few would install these parts but very few Z28's were sold with headers and cowl plenum.......period. The original owner of my white '68 Z28 ordered both options and Reedman would not install the parts, and they were, and still are, a very well known large dealership in the Philadelphia area.
Happy New Year,
Jerry