Are you talking about the receiver the part that bolts to the truck frame, or the drawbar the part that slides into the receiver. If talking about the receiver they are purchased to fit your particular vehicle, if you are talking about the drawbar they come in many different lengths and drop heights, usually it takes the tow vehicle and the trailer to get the proper drop height to pull your trailer level, this is very important. Other them that I have no idea what you want, king pin box threw me off.
To make this easier for you, you should get your 2" receiver installed first if not already done. If you are buying a Scamp
or any trailer the manufacture will know where the top of the trailer ball needs to be to haul the trailer level, Mine was 22" measured from the ground to the top of the ball, so you need to purchase a drawbar and a trailer ball that when plugged into the receiver the top of the ball is a 22". Now there is another factor here you have to take into consideration, if the trailer has a tongue weight
of say 400 lbs. then you will have to get that ground distance of 22" by putting 400 lbs. at the rear of your tow vehicle and see how much the vehicle sets down with that weight
on it, two 200 lbs. guys should do the job. Now all you have to do is figure which drawbar you need to get so the 22" will still be there when the trailer is hitched to the tow vehicle. My top of ball is at 23" when unhitched and 22" with the trailer hitched. All vehicles will vary on how far they will set down when hitched, mine only set down 1" with 400 lbs. hitch weight
, your goal here is to have the trailer set a level as possible. If your vehicle sets down too much then you go with a WDH to solve that problem. This is actually very simple, just may be somewhat confusing at first.