I did read somewhere that after the war, so many Merlins were hanging around that the Ministry of Supply dictated that new airliner designs should be Merlin powered rather than pursue the advantages of the turboprop.
"Commercial" Merlins (the 600 and 700 Series) have what is known as 'Transport heads and banks', a changed design being more suited to long flights at stable power settings.
Quite a number of Spitfires and P-51's flying these days will have these fitted, most likely down to availability, but there's also the theory that the Transport units are a bit more robust, being of a later standard and all that.