It would seem to me that the simple timeline would negate Longboard design being influenced by 666's.
Longboard surfing really took hold in the 1960s with the introduction of the Malibu style of boards and has progressed from there. There were a number of years where no design improvements were made, that was from the introduction of the "shortboard" till the early eighties, and then the modern longboard was reintroduced and steamed ahead from that point.
666's were really only introduced to Australia about 2005 and were big heavy boards that were not very versatile. Since that time there has been rapid development in both surf style, all round and downwind/ race boards.
Given the different turning styles, standard positions and centres of gravity of 666 versus longboard it would seem to me that longboard would influence 666 rather than the opposite.
Just my humble opinion though.
ET.