HELP! transmission case repair?

I'll tell you what I have done in that exact case on a Chevy transmission though. V out the edges of the case and the broken part and clean the paint off and clean trans and part with brake or carb cleaner really well. Screw a steel 1/8" NPT fitting into the hole but not tight, place the broken part in place on the threads, weld it up and you won't even have to rethread. The melting point of aluminum is so much lower than the steel that it will fill in the threads at the vee and leave the steel fitting intact. Once welded to satisfaction, remove the fitting and you're good as new and ready to screw in the original fitting.