After weeks of struggle, I finally have a new seat belt. Even the mechanics had a tough time getting the bolts out. They got one out by applying heat with a blow torch, and then turning it with a vise grip, but the other was in such a bad spot that they couldn't get it with the torch. They attempted to drill it out, but the steel was too hard. Eventually, they welded a nut onto the bolt head, and turned that to remove it. It was a tricky weld, since the space was so tight, but it worked out in the end. I'm very glad that's over, and am hoping that it is a long time before I have to do it over again with the passenger side.