I had a short box before I got the truck I have now. The camper dealer installed the hitch, and I didn't check it's location till some time after they installed it. I think it was 2 - 4" behind the rear axle. The truck seemed to porpose more than I thougt it should when towing. At the scale, the camper lifted 40 lbs off from the front axle, so it wasn't all that much. On my LWB truck that I have now, I installed it 2" ahead of the axle, and it works very well. I can't tell if the difference is because of the LWB or the hitch being better located, or (most likely) both.
My recomendation would be to try to locate the hitch right over the axle on a SWB truck and get a slider hitch. Also, if you can, do the hitch install yourself. You will do a better job than the dealer, and it's not all that hard. Just measure 3 times and check again.