Hello Justin, welcome to 3WW.
I wouldn't weld your starter to the case. I would either drill out and tap the threads the next size larger or helicoil it. If you weld the starter housing on and then something goes wrong with the starter you will be up the creek! Plus, I'm going to take an educated guess that your starter housing is probably steel and your engine case either aluminum or magnesium, which means welding the 2 components together would be impossible.