5k lbs weight limit on the PC hitch. GC over that.
The label on the hitch on my 2010 2551 has two ratings;
For weight distributing type hitch: 6,000 lb max trailer wt and 750 lb max tongue weight
For weight carrying ball mount type hitch, 5,000 lb max trailer and 500 lb max tongue
Not sure why the hitch type allows for different trailer weights, but maybe towing a vehicle with essentially no tongue weight would be ok up to the 6,000 lbs??
I would definitely be talking with PC before I did that: reasoning is that most of the frames have been sectioned and plugged. On the 2007 I had, parts welded in were not full length welds, so by default the new parts were not as strong as the original frame, from which all those numbers derive.
Regarding the frame extension reinforcement not being full length welds:https://www.fleet.ford.com/truckbbas/non-html/Q18.pdf
This Ford document describes the recommended procedure to lengthen the frame and the preferred method is skip welding and specifically leaves some areas of the reinforcement piece unwelded to allow flexing and to eliminate stress risers that could lead to cracking. The document describes in detail the disassembly, lengthening, and reassembly procedures, very interesting if you like that sort of thing.