I guess I am not 100% clear on what happened. From what I get, you replaced multiple starters and they all ground when cranking. After inspection, you then found the crack and you are thinking this is the problem.
If I have to wonder if the starter cracked the block from binding or if the cracked block let the starter twist around. I have to wonder if the first starter cracked the block. Any thoughts?
I would give the welding a try or you might try to braze it. Welding will make the area brittle and prone to crack again. The brazing might just hold and it will not make the block brittle.