In addition to the possibility that the negative cable is not the one that should be going to the starter (I believe early GMC's were positive ground), I saw an article recently talking about the same type of problem you're having. In that case, the battery was also hot and boiling, but it cooked the ground cable as well. The cause of the problem was a poor ground. The owner removed the cables and all ground straps and cleaned the terminal sites. That eliminated the problem.
