If I understand you correctly, it sounds like the threaded inserts or anchors are not the problem, you just need to remove the broken stud and replace it with one that matches (imports have different threads than real Gibbies). Get a screw extractor kit which is a small reverse-threaded corkscrew tool along with the correct drill size. Drill a hole in the center of the broken stud and use the tool turned counterclockwise to remove it like any other broken bolt. The tool looks like the nose of the sub on 10,000 Leagues Under the Sea. Much easier than it would be on a car engine, since you're dealing with brass or pot metal and not steel.