I strongly suspect the root problem is in the starter mounting holes location in the block. GM has had this issue before with blocks machined across the border. This can result in the starter drive not meshing deeply enough into the ring gear. After some period of time the gears wear enough the starter drive "climbs" on top of the ring gear teeth and something has to give. Starter bolts break, the ear on the block breaks off, or the starter nose breaks.
It can be determined if this is actually the problem by "blue printing" the gear engagement pattern and cranking the engine for a couple of seconds.
If the engine has the classic "chrysler" cranking sound that can be a clue that the tooth engagement is suspect.
In some applications the gear pattern can be "blue printed"
without pulling the engine.
Hope this helps.