I picked 1943 for a reason. They just didn't "turn off" the production lines for the P-40 and P-39 in 1944, it took a number of months to wind things down. Perhaps if they really needed more P-51s (with engines from where?) they just would have shoved hundreds of P-40s in various stages and piles and piles of parts out the doors, followed them with the jigs and fixtures and tooled up to make P-51s. How many months where
nothing is coming out of the factory?
Nov 1943 was the last time Curtiss built over 400 P-40s in one month. Dec was 200, Jan-March bounced from a high of 283 to a low of 241, April saw 203 and and Sept saw 202, several months during the summer saw numbers under 100 per month.
Bell was more sudden, they dropped from making 4947 P-39s in 1943 to 1729 in 1944, Aug saw the last 13 P-39s, of course Bell built 1725 P-63s in 1944. Curtiss was building ????? after the P-40. What do you do with the workers? The Curtiss plant was hundreds or thousands of mile from the other aircraft plants. Maybe Curtiss made B-29 ailerons as a subcontractor?
Planning was looking several years down the road, then "stuff" happens. Sometimes good (only took a few months to stuff the Merlin into the P-51) sometimes bad (Continental engine and few other US engines, Sabre engine/Vulture).
Hawker got "stuffed". They had 3 airframes flying with 3 different engines in 1940/41. NONE of them were combat ready in 1941-42. The Sabre is questionable for at least part of 1943(longer?). The Centaurus finally shows up in 1945. 3-4 years late.
Of course not only did Hawker get stuffed. So the RAF (and maybe Westland
The RAF needs something, anything, to tide them over, even if they want to build more Spitfires that is going to months (over 12) to get any real numbers and you need some sort of fighters in NA, in the Far East and to send to Uncle Joe and you need them NOW!
Meetings with Napier could have been done with wire recordings or playing phonograph records, "We are working on it and everything should be fine next month" on repeat. repeat. repeat.
The Hurricane II was never going to be in front once the 109F showed up. Then they stuck the four 20mms in (bomber interceptor?) and then it was ground attack pretty quickly.
With the Sabre (Vulture/Centaurus ) fiasco what choice did the RAF have? They were buying (begging) all the P-40s they could get.
and again, it takes months, and months, to change factories over. Even in 1941/42 (?) Curtiss had 7 parallel production lines running. 7 complete sets of tooling.