I am a newbie part timer and am somewhat confused about what the P1400 is for. It seems like the same length as the P1200, only the roof is lower, which seems to leave less space on the shelves. And it doesn't have the curvy fiberglass top in the front. Also, what does the HUD stand for in HUD Truck? Or at least thats what I've heard the straight trucks called so I assume its a acronym.