P-51 with it’s water cooled engine was always a poor choice for ground attack. A-1 was excellent until heat seeking MANPADS got good enough that they could engage it from head on. I’d be inclined to think that a radial mounted in a pusher configuration would be hard to beat in terms of loiter time and ability to go low and slow.
The first P-51s fielded were actually the A-36 dive bomber.
A rule of thumb.
Extend your arm with thumb out, like an artist.
Sight your thumb over the aircraft.
If your thumb covers the aircraft, save your ammo. If it appears larger than the thumb, go for it!
Small arms only.