I was told that the wheelbase modification raised ratings from 13,000 (on the metal tag, in the cab) to 21,000, I think because of how the weight is distributed between steer and load axles.
The hydraulic brakes and springs are the same as on the 5 ton (HD) version, (judging by part numbers) but with 16 inch rims VS 19 /22 inch on the 5 ton rated. (curiously, Isuzu doesn't rate any of the NPR series for towing, that I could find anyway)
My insurance had a bit of heartburn accepting the 13,000 rating with non commercial plates, DMV didn't have a problem. A call to my upfitter laid all that to rest, I now have non CDL plates, no license restrictions, as well as cheaper insurance here in Floriduh Really glad I didn't try to do the cut down on my own.
I do know that my current 5er weighs 6800 lb. and I usually run the pin forward of the axle C/L ~ 5 inches.
At ~ 3 inches forward of C/L, occasionally, road surfaces or conditions will induce short bouts of chucking. (Heavy rain and headwind is the worst, never been in snow/ice. a light touch on the brake controller will usually stop it) When backing, I normally slide the receiver rearward, as far as I can, ~ 3 inches behind the C/L. Sitting right on the steer axle, it feels much like spotting a trailer with a fork lift, very easy to over steer.
I've only been stopped 1 time, down stream of a weigh station, for no visible DOT numbers, Gotta get the "not for hire" stickers, when the cop saw the Class 4 on the registration, he smiled and waved me away.
Sorry for the book,
Paul in da swamp