Just Stuffing a Merlin XX or 61 in a Mustang one only gives you a test aircraft. the RR Merlin was heavier than the Allison and IIRC the wings had to be moved forward several inches to get the aircrafts CoG back to within limits that gave a performance and handing suitable for combat.
Merlin XX weighted about the same as the V-1710 + two HMGs installed under it - basically no change in weight distribution. Unfortunately, there was no such thing as Merin XX/V-1650-1 on the Mustang fuselage, let alone many hundreds of them like it was the case with P-40F/L.
P-51B introduced fuselage deeper by 7 in, longitudinal position of wing remained the same. Heavier engine was counterbalanced by heavier radiator system, that now included a radiator that served the intercooler (similar 'trick' was used on Spitfires and Bf-109s), as well as with introduction of the fuselage tank by the P-51B-5. Mustang X featured intercooler under the chin, it was still working fine.
It is really too bad that powers that were didn't insisted in turning the Mustang X project (5 total modified from Mustang I) into a pattern for actual combat aircraft, that should amount to many hundreds by May/June 1943.