Are these the wheels you are talking about?
http://s1030.photobucket.com/albums/...t=38bd228e.jpg
They are 16.5x6 on 6 lug. I have never seen another set and I got the set of 5 with the '67 chassis i am using. Not sure where to find a set of originals but Stockton wheel works out of California makes new ones. They are the Heavy Duty Truck - 20 Series at this link:
http://www.stocktonwheel.com/4wd_offroad.php