Interesting to note the following from the American EAA Forums (November 2011):
"The piston DC-3 is a single pilot aircraft by type certification. HOWEVER if your DC-3 has been modified to the 1830-94 or the 1820-76 and any Higher horsepower engine that includes a geared rudder tab, you will have an FAA approved flight manual that mandates it a 2 pilot aircraft."
The FAA Flight Manual mentioned above is actually written by an American Insurance Company back in the 1950's and that is the manual that we have to work to in Australia even though there are numbered pages missing from that manual!
The DC3 appears in some recent CASA documentation also as a Single Pilot aircraft, if by chance I am reading the legislation correctly!. .