Well.... That requires there BE a Packard Merlin in the first place. Which was a direct result of the improved performance of the British Merlin experimental version of the P51.
And it took quite a little while for Packard to sort out the redesign and manufacturing of the British Merlins to US production. Things like screw threads are different, just for starters, and in the UK Merlins were far more hand-built than US engines were, so ...
If Britain sees that war is inevitable when Hitler moves into (the rest of) Czechoslovakia in March 1939, and a study shows that Merlin engines are going to be in desperate short supply, and so they start talking with Packard by, say June 1939,
Maybe have the engines be 'built' in the Windsor Ontario plant (see, e.g.
http://packardinfo.com/xoops/html/uploads/newbb/177_4d0b7473ad2b4.jpg)
If I remember correctly, there was an abortive attempt to have Ford build the Merlin first, and they just couldn't do it.
So. Start earlier. Go straight to Packard. Maybe you could have a Packard Merlin ready in time for the first P-51 (NA-91)s.