All of the bumper ends I have encountered have been rusted out internally but with the rubber intact. So my solution has been to carve everything out of the pocket in the bumper so that all that's left is a solid piece of rubber. Then fit a plate of 1/8" steel to that pocket. You can buy this stock at Lowes' in 3' sections.
Cut off a 6" piece, and bend it in the middle approximatley 10 degrees (bend to fit the inside surface of the pocket in the rubber). Cut a 1/2" hole where the stud should be and fit a 3/8" by 1" SS carriage bolt into it. Now work that assembly down into the pocket. Seal it up well with Butyl or Rubber and don't forget the little hole on the outer side. I've done this with all four corners of my spider and they have held up well.