Originally Posted by padlin
From what I saw on my hitch install instructions, the fifth wheel hitch is a fixed distance from the rear of the bed irregardless of the bed length. If you had the 6'6" bed you'd have 3' 6" clearance between the trailer and the cab etc.
That makes sense, because most modern trucks are the same from axle to rear bumper regardless of the wheelbase - all the change in length between variations of cab and box length are within the wheelbase. The frame mounting holes used for fifth-wheel hitch brackets are probably in the same place relative to the axle (and rear end of frame) regardless of cab-to-axle spacing. The distance from pin to truck bumper corner is limited by the reach of the trailer - keeping it consistent is good for predictability of fifth-wheel trailer fit.
That specific dimension doesn't quite make sense to me - the normal location would likely be more than 3 feet from the back of the box, because it is normally slightly ahead of the axle, but maybe it's really only 3 feet.