Check GT-racing too (
www.gt-racing.com), don't know about the downforce stuff but if you call gt-racing, Hank may be able to answer some of your questions... try to find a ducktail with a vertical reinforcement (like the one Mark has), from time to time in the Pelican classifieds you will find ducktail and I know of someone that has an original ducktail for sale (not cheap!).
Also you may want to check with Pelican Parts, I'm sure that they will carry ducktails.
No affiliation with any of the mentioned suppliers.
regards