it my understanding that the machine gun mounts on the GMC's were meant to be used in case of an attack by aircraft. If this is the case then only a couple would be needed in a convoy. They were not meant as an offensive or defensive weapon against infantry, however probably some were used that way.
I would suggest that the ommission of winches on some trucks was more of a case of conserving steel and costs as not all trucks needed the winch, when others were traveling with them. All tippers had winches, but then they are construction trucks.
I have a friend that was a South Pacific combat veteran with the 2nd Marine Division and he said that they mounted pedestal mounts, similiar to the jeep mount , in the rear of the IH trucks , right behind the cab, with the .50 caliber machine guns and used them against the Japanese in an offensive manner.
We all knoiw that in war time ingenuity in the field can mean the difference between victory or defeat and trucks that were supposed to be support vehicles can be quickly turned into weapons of war.