When the DC-6 was certified it was approved for a crew of two. Airline unions made it mandatory that there was a crew of three when operating for hire. Crew of two was ok for ferry and that has been gone for years. The FE was really an afterthought on the Douglas DC-6 as there was no proper station for them like the Boeings or the Lockheeds. There was also a provision for the 1820-202 powered DC-3 to be operated under Part 135 single pilot so long as the cowl flaps were converted to fixed position and you may have had to do something with the Gear system as well. Last one I know of was Harry Clark's out of Hawii.