Tony,
Wasn't the M4 only used in a few B-25G's while all of the H models had the T13E1 75mm. I'm not totaly familiar with either, but from what I can tell the T13E1 was basically just a lighter version of the M4. Where there any other differences?
My point wasn't that the B-25H would make a poor anti-tank weapon, but that it would make a poor anti-shipping weapon. I don't think it could hurt a destroyer and certainly not a carrier. Also I'm not sure, but I think the sighting system of the cannon ment that there was NO bomb sight. Perhaps they were using bounce bombing techniques with these aircraft.
It would be pretty good against a PT though or any of the LVT's, M-16, M-8, and M-3. Probably not too bad against the Tiger or Panzer either.
Spritle