I've welded a bunch of aluminum transmission cases. Some are easier than others. 4043 is the correct rod for cast aluminum. Don't be surprised if the weld has a bunch of pin holes in it where the weld meets the case. If the part that is broke isn't very large I would toss it and just build the weld up and mill it flat and redrill the hole instead of welding the broke part back on. It would be stronger this way.