Just make sure the battery you are installing meets the minimum wieght on the ID tag, and you are OK. If the battery is smaller than the battery box, you are OK as long as you meet the minimum weight on the ID tag. IF the battery does not meet the minimum wieght, and you are not able the purchase the correct size battery, I have seen steel plates added inside the battery compartment to meet the minimum wieght.

Also, if you battery is narrower than the compartment, put some sort of a spacer(wood blocks will work), in the compartment to take up the space. Make sure you install the battery as close to the rear of the compartment as possible.

Again, whatever you do, YOU MUST MEET THE MINIMUM WEGHT ON THE IDENTIFICATION TAG.

I can't answer as to why the truck with the "C" compartment needs a heavier battry than the "B" compartment. I leave it up to the enginneers at Crown to determine what size battery goes in a truck. The frame may be different to accomodate this difference in battery weight.