This is from the public domain FAA aircraft registrations database via
www.landings.com
Although these records are notoriously unreliable they do reveal at least one example of a PT6A powered Cessna 421 :
N-number : N101TA
Aircraft Serial Number : 421C0331
Aircraft Manufacturer : CESSNA
Model : 421C
Engine Manufacturer : P & W
Model : PT6A SERIES
Aircraft Year : 1977
Owner Name : SUNFLOWER AVIATION LLC
Owner Address : 300 DELAWARE AVE FL 9
WILMINGTON, DE, 19801-1607
Registration Date : 08-Dec-1999
Airworthiness Certificate Type : Standard
Approved Operations : Normal