Heres some problems I see with this.....
#1: you would get two kills, if it takes two people to work a towed gun (M3/ SdKfz 251 driver, and the actual driver), instead of having to kill both to get a single kill.
#2: Also, you'd have to make it so the gunner only has, say, 30 shells with him unless HIS halftrack stays within 20yds of him, then its 50-60 shells.
#3: this would be good for defense, but not really good for offense. And for defense, you have the problem of the gun being able to cover only what he can see from where he was placed by the M3/251 (well, from wherever he rolled himself to within a 50ft radius), where as a tank can cover roughly 4 times that much gound, and do it more effectively (he's got armor. even a D3A will pose serious worries for you)
#4: even the low bushes we have will get in your way, unless you put yourself on a hill, which negates the only purpose of having a towed gun in AH (smaller = harder to see).
#5: Self proppeled AT guns, such as the M3A1 GMC (M3 halftrack with the gun from the M4A3(75) mounted on it, and the SdKfz 251/22 (SdKfz 251 with a Pak-40, the same gun as the Panzer's, mounted on it) would be much more effective, since you don't have to rely on someone else to move you, you carry your own ammo, and you will be able to see over the small bushes while retaining a smaller outline than a tank.