The are hardened bolts, but for a 225...
Buy a slight oversized bolt with a self tapping thread. Grind the head down so that it is shallow like the one that won't hold anymore.
Thread it in until tight and go.
That I'm aware of, a slight weight imbalance shouldn't cause much trouble. If you pick up a new vibration, you'll know where to look.
If the car is not driven hard, it should do fine. I had one like this on a street 340 for a while, no trouble.
Only way to fix correctly is to get another boss welded on the converter, or replace the converter.