I did the same. If I had my body lift on when I broke one of the upper rear bolts I would have been able to attack the problem from underneath. I used a hole saw and cut an oval. Cut the welded nut and bolt off with a diamond cutting wheel on my Dremel. Inserted a new bolt and nut and that was that. Then I bought a small piece of sheet metal at Home Depot and cut it to fit. Primed it and placed it over the hole and glued in place with gutter seal. It is under the carpet so not really a big deal. Pu plenty of anti-seize on all bolts, nuts etc.I lifted my body, cut the welds with a Dremel, and bashed them out. Welded some bolts in to replace them. I was doing a body lift at the same time, so it made sense. If you run carpet, @derekmac's method would work well.