There are many differences in the hoods alone, not to mention the cabs. The
"A" hood has round headlights, rounded fenders, a raised triangle on the top of the hood and the top has more slope to each side where it meets the grill.
The "B" hood has square headlights, more squared off fenders, a straight raised rib on top of the hood and the grill is more squared. These are just things I can remember off the top of my head. I'm pretty sure there's a few more differences.