Tough call, but my guess is it isn't legit. Either way though it's had a lot of parts swapped from another car. It does have blacked-out window moldings and light bezels which suggests KC. But the targa band has chrome which should be black. Other things to check for, besides the buck tag which I assume is missing:
- KC brake ducts
- rear sway bar (although maybe all 78's had them?)
- bumper molding pinstriping colored to match KC pinstriping
Also, check the wheel spats. From the factory they were attached with aluminum pop-rivets. Check behind the spats too if you can - this area is likely rusted through.
Are you thinking of buying it? It's missing a lot of hard-to-find KC parts, so I wouldn't pay any more even if it does turn out to be a real King.