My Piper Information Manual for the Navajo/Chieftain states the MCP for the PA31-350 is 2575 RPM. At ISA the MAP would be 32-31 inches between 5k-10k.
With reference to the unfortunate Whyalla Airlines accident, I recall that the engines were being operated with reference to the engine manufacturers recommendations and not the aircraft manufacturer recommendations. As I understand it either is legally okay.