I had this same trouble in places on my 47. Some aareas were pushed 3/4" apart with rust and dirt. I decided to remove the ones at the back. I used a 4" angle grinder to remove the top, and a large punch to get the slug out. These rivits are put in the frames hot. You will need an air tool and a torch to reinstall them. You can replace some rivits with common grade 5 bolts. Most of the time rivits are a little bigger or smaller than a bolt. If you use a bolt make sure that none of the threaded part is in hole, as it will cut it and not remain tight. I use a longer one and then cut off the extra length. I also found the triangle brace on one side was rust almost completely though, and wouldnt have know it if I hadn't of remove the rivits. Another place this happens is on the spring stops on the outside of the frame. The process is similar. just remove, clean it up and reattach them.
