Actually I just had all of my tires, air bags, and shocks replaced, and when we got the rear tires off it showed that the strut rod mount had cracked the frame where it attached. I was not broken loose, but had to be flexing with the crack. Luckily the crack was limited to the sidewall and did not go as far as the top or bottom rails. Had it V'd out, drilled at the ends of the cracks, and then welded back up and ground smooth. Added a plate to the frame, drilling all the frame holes into the plate. 10 total, not just the 4 that held on the strut rod. Used grade 8 bolts in all the holes and then welded the perimeter of the plate to the frame. It is over 1/2 an inch thick there now and the bolts keep the center from flexing.
Looking at your break, I now feel lucky that it was the opposite end that failed on mine. I could not see the crack prior to having the wheels removed. I also don't know how common this problem is, but I will be keeping an eye on this in the future. Good luck with your repair, sounds like a big job where the right tools are needed.