A quick run-down on the P51s:
P-51B Compared To P-51D:
P51B is lighter, climbs better, rolls better, turns better. The downside is that it has 2 fewer guns and less ammo that the D-stang. The D-stang seems to accellerate in a dive a bit better, plus it doesn't compress nearly as bad as a B-stang does. The B-stang has a blind spot behind it that the D-stang doesn't. The D-stang can carry heavier bombs. The B-stang will suffer drag penalties if you carry rockets, whereas the D-stang will be "clean" after you've fired all your rockets. The D-stang "blinks" beyond icon range, letting everyone know that you're a D-stang and gives them a chance to runaway. A B-stang can close the gap before the icon gives it away.
I prefer to use the B-stang as my fighter plane and the D-stang as my bomber. The B-stang is just a better air-to-air performer overall. All things being equal, a B-stang should "pwn" a D-stang in a fight once the airspeeds drop below 400ias.
The D-stang is the better air-to-ground aircraft and a better bomber interceptor (just remember to stay off the tails of B-17s and B-26s and instead use deflections shots and make high-speed swooping passes). Its also a better Boom-n-Zoom fighter in that it can put out more punishment in a snapshot than a B-stang can.
The B-stang will start to shudder just over 400ias, and will compress at around 450ias. The B-stang will slip sideways before it starts to shudder, an indication that speed is about to become a problem. Listen to what the airplane is telling you. If you feel that sideways slip, drop a notch of flaps, retard throttle (if you haven't already), and kick a bit of rudder. If you find yourself in a state of compression, use the trim keys to get your nose up.
I set my convergence at 450 but usually don't start shooting until 350. The reason why I have it set to 450 is so I can shoot from a farther distance and land my shells, increasing the duration of my firing window. I typically carry 50% gas and drop tanks and avoid engagements until I'm over 10k. The D-stang can have a vicious stall if it gets too slow and may need that alt to recover.