Someone, not me, has just listed a rudder off a scrapped 1977 6.8. I don't know the person or whether they have any additional parts.
http://cgi.ebay.com/ebaymotors/S2-S...5805092QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear
http://cgi.ebay.com/ebaymotors/S2-S...5805092QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear