For future clarification, the difference between an A-36/P-51A and a P-51B/C is not only in the nose and radiator scoop/doghouse areas (and of course many other small details), but the whole fuselage is narrower along the vertical/z-axis on the Allison-engined variants than the Merlin-engined variants, since the lower longerons are designed and spaced differently between these types. The wing on an Allison-engine variant (A-36/P-51A) actually forms the floor in the cockpit, where as because the wing is mounted lower on the Merlin-engined variants, floor panels were added, compensating for the difference in distance.
On the A-36/P-51A, the lower longerons jut-upward from the lower engine mount locations, so that the wing can be mounted high, making everything as streamlined as possible. On the Merlin-engined varaints (P-51B/C/D), the lower longerons were redesigned to carry almost straight back from the lower engine mount locations (same locations as on the A-36/P-51A), so that the wing would be mounted lower - this was so that with the mounting of the vertically-larger Merlin, the lower cowlings/forward carb ducting wouldn't dip below the line of the wing, blocking airflow to the radiator scoop. By mounting the wing lower on the B/C/D, there could be a clean line from the carb intake, down over the bottom of the engine cowlings, over the bottom of the wing, and up to the radiator scoop.
A-36/P-51A fuselage (note lower longeron design):
P-51B/C fuselage (note lower longeron design - the firewall is the same between the A-36/P-51A/P-51B/C, but was shorter above the upper longerons on the D, requiring different upper cowl panels than the B/C):
Again, a P-51A fuselage...
And a P-51B/C fuselage...
