Brownells sells a low temp solder that melts at 275 degrees. Or you could put some heat control paste on the ribs to keep them cool. I have made small spot repairs on ribs by making heat sinks to keep the area past where I am working from getting too hot. Used dry ice on one job with wet cotton to make a safe area which heat I was using could do no harm to the rest of the gun. It is all about keeping a minor repair from becoming a major disaster as you have said. But minor repairs can often be done with what you have if you take precautions to keep the bores and ribs cool.
