For starters (no pun intended) your friend that welded on the block probably only made the problem with the damaged boss worse. Welding cast iron has a success rate somewhere between zero and a very large negative number. In spite of how it looks if you put it to a Magna-flux test you'll find more cracks well into the casting.
Bolts, GM starters don't use ordinary bolts, they have a knurled pattern resulting in a slight press fit in the starter mounting boss. This prevents movement between the bolt and the boss. Ordinary bolts cannot hold the proper alignment regardless of how much torque you put on them or how high their grade number is, they just cannot be made to work.
My expectation is that the block is damaged beyond repair, the Root Cause was probably incorrect mounting bolts allowing the starter motor to snap against the bolts causing them to twist and breaking the casting, or the casting was already broken/cracked thus allowing the bolt to twist under starting loads with the same end result. If the bolt can move in its threaded hole even if it's the proper knurled bolt, this whole scenario will take place.
You have to have at a minimum a good crack free casting with proper threads in the hole AND the PROPER GM factory bolt.
As for the flexplate, look for uniform run out longitudinally and laterally. Which is to say the ring gear must not wobble in the fore and aft direction along the length of the car. Additionally the ring gear must be concentric as the crankshaft is rotated. Any of these problems can also put a twist on the starter that can lead to cracking it or its mount on the block.
Also if the mains have been aligned bored, it's possible to have moved the crank up-wards in the block such that the ring gear is too far from the starter pinion and it now develops a significant impact moment when the gears take up the slack, thus snap twisting the starter on its mounts resulting in damage to the block casting. The only repair for this is to mill the top of the starter mounting boss to bring the pinion closer to the ring gear. When properly engaged the wire diameter of a large paper clip should just be able to pass between the tip of ring tooth and the root of a starter tooth when they are engaged.