I believe that Lufthansa cargo had a daily 747F from Frankfurt to ? USA during the 70s and 80s on behalf of Volkswagen, so in effect that was a car ferry. I'm sure that they used both decks or at least a palletised double-decker.
Also, some of the Arab princes have been rumoured to have transported cars and horses around in DC-8s/747s in the past
Regards
SHJ