They showed this on the TV show "Trucks". What Stacey did was, he put the bolts in and a washer on the backside of the bumper right against the bumper that fit tight over the bolt. He said if the bumper is curved too much you might have to curve the washer to follow the contour. Then he welded the washer to the bumper (from the inside). Once he did that he welded the bolt to the washer and built up a nice strong weld all the way around letting cool in between. Then he ground down the head and spot welded unitll he filled it up nicely, then ground them down again.