The required OEM rear support brace and correct OEM GM knurled starter bolts would have prevented all this. You get these from a GM dealer parts counter.
If / when you crack /strip out the end starter bolt hole on the block,
The fix is to converter to the old 1955 1956 era way of mounting the starter motor
on the first 265cid sbc's, with one of these starter mounting adapter and a 1955 era style starter.
Replace a chewed flex plate gear while you are in there.
PS this adapter also needs the support brace on the end of the starter motor.