Some turboprop options for the PA31 series.
Piper T-1040
Not really a Chieftain or a Cheyenne, sort of a half way aircraft. Imagine it 11ft longer and you have the T-1050.
EMB/NE-821 Caraja Conversion
The basic PA31-350 (EMB 820) converted to a turbine with PT-6-27 engines.