Paper Tiger, C-47s were built new with quite large double cargo doors, each of them about five feet wide and seven or eight high, I'm guessing. They had big external hinges about two feet long, two per door. (I'm not sure -all- C-47s were so configured, but certainly most of them were.)
These doors would obviously have to be removed and the big hole made whole, as it were, if a C-47 was to be converted to a passenger-carrying DC-3.
I'm surprised the airplane seems so unfamilar to the English. Don't you remember all those photos of the 82nd and 101st Airborne jumping out of the cargo-door hole over Normandy and Belgium? (The aft door was removed before paratroop flights.)