How about Castle Bromwich Assembly is stood up earlier - say at full production in mid 1939
Other CBA like factories are stood up under the guidance of Albert Kahn as part of an expanded shadow factory scheme (large modern factories using modern production line techniques in turn making use of lots of single use machine tooling requiring lower skilled workers)
This coupled with an earlier loosening of the purse strings results in an improved over OTL shadow factory scheme allowing for much higher than OTL production numbers 'earlier' - wouldn't even have to take 1%
The Ramp head issue on the Merlin is resolved or never happens - some one other than George Elliott joins RR in the 30s so 'proper' Merlin production is stood up earlier
IMO Merlin production was up to the task, after all Merlins were installed in a 1000+ Defiants, 2000+ Battles, later versions of Whitley (2 per A/C obviously), some Beaufighters and Halifaxes (2 and 4 per A/C) etc, while Merlin was to power some versions of French A/C. Production of Merlin was greater than
combined production of DB 601/605 and Jumo 211/213 in any year, including 1938-41.
As for CB factory getting earlier into churning out Spitfires - invest some of that money at Boulton Paul and Westland in 1938-40 for their production of Spitfires (with Defiant and Whirlwind cancelled). I'd again propose introduction of single-piece ribs vs. built-up ribs to save manhours.
Further about the engines - say, Napier makes licensed HS 12Y instead of the Dagger (= lighter, better altitude power, if not on Merlin level), Allison V-1710 for US and Canadian production, Hercules, R-2180?
As part of the Tizard mission the design is provided to the US and the manufacture of Spitfire's is stood up in several US Aircraft factory's in 1941 and the type is used to equip early USAAF interceptor Squadrons when the USA enters the war - US Production mirrors improvements to the type for the next 3 years (as well as feeding back improvements of their own)
There that should do it
Yes, making Spitfires instead of P-39s and/or P-66 would've make sense. Though, the drawings should've been redone (USA used one projection, UK another), and UK has more lax G limits than USA. USAAC would've also asked for greater range, so we might see a LR Spitfire inn 1941/42 instead in 1944. Four, and then six .50s would've looked nice on Spitfires.
Spitfire will out-climb P-40 and P-39, due to being lighter.