Think of the boarding plank like the ladder of a fire truck, it is pintle mounted and can swing any direction it is needed. You could mount it off the rear, off the front, off the center, etc. In some of my trucks as they sometimes don't have a plank, I just have a loose ladder that is placed in the bed of the truck with a boy to hold it.
This way if I run Drive By Boys I don't have to pay points for kit that I won't use, and this allows me to take more stuff. While if there is a nob with PK in the truck, you can bet I have a plank, especially if I know I am going up against dreadnoughts. I just love it when BA players cry cheese after you kill off their death company dread