Just throw backdate targa in the search function for this forum and a bunch of stuff will pop up for you:
http://forums.pelicanparts.com/porsche-911-technical-forum/908611-78-lightweight-targa-backdate-thread.html
This is a fun one too:
http://forums.pelicanparts.com/porsche-911-technical-forum/139484-ultimate-backdating-thread.html. If you are starting a project be sure to start a thread for us to check out and follow your progress.
