[BOZO]

You won't have much luck welding on the block, but brazing will work being as it is a cast metal. Also instead of a helicoil, use a Keenesert. These are threaded inserts which use the next size bigger threads drilled and tapped into the block then the insert is screwed into the block and the locking tabs are driven into the threads to hold it in place. Helicoils require a specific tap to create the threads and are more prone to pulling out than Keeneserts.
Some starters have a 3 bolt aluminum housing. Sometimes these can be used to drill and tap an extra hole in the block. Also, as posted earlier it is a good idea to use the front bracket (a stock Chevrolet item) to brace the starter whether you have a problem or not.