Cowl tag is not required for registration. It is just a means of identifying what parts were installed in the body by Fisher Body Corporation. An independent and occasionally not any where near the assembly line. It allowed the workers on the assembly line to know what the car should ultimately be, but even that was not carved in stone (as often a customer wanting a red car with a V8 would get a car that the trim tag said it was supposed to have a six in it). So the trim tag is not used for identifying the car.
The VIN tag on the door is used for that. If the VIN tag is missing you are in deep Poodoo in Star Wars vernacular.
You can buy a trim tag on line that has any options you want stamped in it. Any date of construction, any color, any trim level, any assembly plant anything you want stamped on the tag. The same people sell new or tea stained aged invoices and window stickers as well. The only things that are illegal to alter is the title and the VIN. That is a federal felony.