Building the Merlin in the US was indeed a far more complex and involved affair than most people realise.
Whilst the P51’s radiator arrangement deserves praise, don’t fall for the hype. Having the radiators in the wings ala Mosquito is more efficient, which is why several P51 racers had their radiators moved to their redundant wing gun locations.
Also, increasing the Spitfire’s range was far more complex and involved than just shoving in more or larger fuel tanks. Keeping up with German advances such as the FW190 took up most of the design teams’ time and effort; handling and design weights were an issue, as was the later introduction of the bubble canopy’s reducing fuselage volumes. However, the
PR Spitfires were so good that the USAAF used large numbers.