The Super DC-3 (DC-3S) also has the fuselage stretched both fore and aft of the wing. The outer wing is a different airfoil with some leading and trailing edge sweep. Wright 1820's.
It has a post-WWII technology AC and DC electrical system, with some fault tolerance.
The load capacity is increased, as one would expect. The horizontal tail area is roughly doubled.
The fuel capacity is doubled over the DC-3, 1600gal total (the additional tanks in outer wing panels).
Usually has radar.
The tailwheel retracts - that's the good part; the bad part is that in ground handling there is no more pushing the aircraft back and manuevering in restricted parking - tow it forward, or hook up to tailwheel and tow it backwards, with roughly 45deg (i think) tow limits.
Overall, a shame that only 103 were built - and built (remanufactured) from actual C47 airframes, I believe. 100 for the Navy/Marines, 3 for Capitol Airlines.
My parents rode on part of their trip to/from Europe in the 50's a 14 hr. (!) leg one end of which was the Azores (they were Navy officers).