I was thinking of doing the same kind of thing. I bought 2 bumper mounted hitch receivers and thought I would mount them to the bumper at the point where the bumper attaches to the frame (at the tubular steel brackets). I figured this would spread the weight evenly and also be unlikely to "twist" the bumper as the rack would bounce while in transit. My only concern is that I want to then add a little more support to the area where the bumper and the 2 pieces of tubular steel attach to the frame. I also plan on adding a rear camera so I can keep an eye on things as we travel.
My unit came with a small auxillary hitch receiver on the bumper but I would like something more solid like the rack you have.
Spinella- I would like to do what you did but my bumper sits below my frame. Now that I think about it. I could take the bumper off, add the tubes to the bottom of the frame (and run them far enough down the frame to really support the weight) then mount the bumper to the top of the added tubing. It would raise my bumper by about 4 inches but I don't think that would be a problem. I will have to think about that possibility.