In addition to all of the physical problems associated with putting a Griffon into the P-51 there is also the question of where the Griffons come from. According to the Fedden Mission to America Report
View attachment 677206
It would seem that was an either-or problem. The major user of Packard Merlins was the RAF. In particular the Lancaster program was heavily dependent on the US produced Merlins. A wholesale switch over to Griffons as not a practical proposition.