Weld a threaded Flange to each side of the frame, with holes behind each bolt hole. Then flange the end of the tube you are attaching to it, and bolt it on that way. Still bolt on, and will require only minor modification to the frame.
I know "dezertrunner" didn't like the way I built my bumper and said it would total my truck if I were to hit anything, well a few months back I got into an accident, totaled out the lil suv I hit, and only thing that happened to my truck was the bumper got pushed back and busted the grille, one fog lightl and the headlight, got home, pulled the bumper back out and replace the headlight and I was good.
the left upright tube did get a lil f'ed up (where the rim of the suv hit, found a chunk of his rim on my skid plate) so I built my own uprights out of 3/16" and 1/8" flat steel