PDX...this kind of stuff is fun for me, but to not lose sight of you the OP's plight...
Going from a WD 1,000 lb tongue max rating to something like 1,200-1,500 lb isn't going to need major redesign...just enough material (thickness, box section, etc) to distribute that 'extra' weight/loading (force vectors) to a wider portion the frame is NOT rocket science...
Suggest you do consult with a ME/PE for peace of mind and documentation for your own liability (never leave that stuff to chance, IMHO)
Unless you have the mind to and the shop, it might cost a few more coins than me with three different types of welders, a plasma cutter (modifying it for 1/2" plate...think upping the air PSI will do that and not blow the handle/gun to pieces), cut-off band saw, 80 gallon 2-stage compressor, and two 55 gallon steel barrels filled with scrap metal for this kind of stuff...
I'll still noodle this, but DO NOT depend on my comments other than just suggestions, as I have zero skin in your game...just an Internet interest in this kind of stuff
HAPPY EASTER !!!!
