Purists and car show judges won't appreciate it, but of course your Mustang will remain a '78, tecnically, at least. I once owned a '71 Camaro that I ended up replacing about 85% of the sheet metal and almost all the mechanical parts on it, but it still looked like a '71 Camaro and I sold it as a '71 Camaro.
Fortunately, I came to my senses and wisely used the money from that sale as a down payment on a 1990 Mustang, which I still own, so the story had a happy ending.
