All of the DC3s that I flew (P&W R-1830s) had fixed cowls, it was explained to me as a young sprog that -
(1) The variable cowl flap system was unreliable, erratic, and required a lot of maintainance, and
(2) Unnecessary in the Temperate / Tropical conditions in which we operated, with the fixed 'optimised' setting.
The latter was true, we rarely had any CHT problems. Only once did I ever see the use of 'Auto Rich' necessary during cruise, and 6 or 7 times had to resort to 'Emergency Rich' during prolonged climb at high power to 16,000 feet or so to avoid the New Guinea hills. In both cases, use of the Mixture control worked well, and kept the CHTs within limits.
We were obsessive in the use of 'balanced power' (not much less than Holding Power) during descent, and never had any problems with low CHT.
The only twitchy part of the engine controls was the Carby Heat, with only a small tweak of the control resulting in large variations in intake temperature.
Regards,
Old Smokey