The dually pickup bed was actually a factory item from Chevy (Model 4104--Utility Express):
http://www.brads41-46chevys.com/id22.html Unfortunately, there weren't many produced and since these were real work horses, they were used hard and not many have survived.
I was lucky enough to accidentally find a dually pickup and have done a lot of research on them. I've collected some literature and sales books that actually show the dually pickup. I also contacted Jim Carter but he said that his and mine are the only two he's heard of.
There is another member here, Bob Galet, in Pennsylvania, who found a bed off another truck and wants to make a 1939 dually, but the other truck had single fenders, so he's also looking for the duals.
Your best bet might be to do something like Shawn did and build your own. By the way, he's also posted his pickup on the Stovebolt site:
http://www.stovebolt.com/gallery/maher_shawn_1946.html Bob
