I would have to qualify and say yes and no but ultimately claim that it could have been done.
The Merlin engine P-51B took nearly 2 years to get into production. Surely in that time the turbo Allison could have been fitted. Now in terms of the space issue.
1 Q: Where would the intercooler be fitted?
A: The same place the Merlin had its water cooled intercooler: behind the engine.
2 Q: Where would the turbo supercharger be fitted?
A: The same place the Merlin had its mechanical supercharger: behind the engine and underneath the supercharger.
Of course the assembly would have been somewhat larger, but its hard to imagine that it would be so much that it could not be dealt with by a slight nose extension of the P-51.
The twin Mustang was for instance some 18 inches longer. The P-51B had nearly every panel redesigned compared with the P-51A so given the redesign this was not much to go a little further.
The problem may have been that the NACA's advice to the Army Air Corps which was that the turbo supercharger should be kept separate from the engine connected by ducting. Integral turbochargers, which were integrated into the engine and shipped in a single package would have to be used with such a P-51 turbo Allison.
The advantage would likely be
1 The same top speed, likely higher speed at high altitude, say above 25,000-30,000ft
2 Much better cruise fuel consumption and therefor escort range.
3 Higher service ceiling and climb rate at high altitude.