If what you are saying is how do I beef up the mounting points for my stock bumper, I think you are wasting your time. That stock thang is a piece.
I would highly recommend ripping it off and putting a nice steel one on though. You are right on with mounting it to the hitch.
If this is what you are going to do, think of sandwiching the frame rail in between the hitch and the bumper mounts.
As in, your bumper mounts will be square stock slid into the frame rail and bolted throught to the hitch.
I even welded some plate perpindicular to the square tubing so I could use the original bumper tie in points (4 bolts to bracket).
Then just put whatever the hell you want on the square tubing for your bumper. I used 4x6 or 3x6, cant' remember.
If this isn't what you were thinking, I wouldn't mess with it. Seems like a lot of work for nothing. That stock bumper is going to get all bent up before the mounting points fail.
Just my opinion, hope yer helped out.