Even though I bought a 3/4 ton HD truck when I decided on a large TT I looked at the factory class 3 hitch. Borderline at best. I replaced the hitch with a class IV, the correct load capacity.
If you cannot afford to trade (would be my first choice), I would go with a very reputable welding/hitch shop to build it for you.
My thinking, The thing is your factory hitch may work great for you until you really need it, diverting an accident, bad weather (wind), something that causes bad sway you need to correct, then the factory hitch could rip out of the frame.