HI,
The engines sure where of Packard design.
My Dad was mechanic (and rear gunner, no below decks on patrol, and rear gun was next to engine hatch), and always stressed that these where not copies or the Merlin. Different engine altogether.
Will be building a model of one of these boats myself, soon as I have enough info to recreate the boat Dad was on while in the NZRAF in the Pacific during '44 and '45. Dad's was also used to supply and transfer coastwatchers during the push up the islands.
Best of luck with your build
kiwi