It's a 305 from an 80-85 truck w/the staggered bolt pattern on the starter. Its been having starting problems intermittently for about two months. I rebuilt the starter (the brushes were worn down almost to the armatures), then replaced the battery today because all it did was clicked. I went to start it w/the new battery, and all it did was made a grinding noise. Further inspection revealed a broken nose cone. Could starting it this morning with a fresh rebuild and crappy battery have torqued the crap out of the guts of it, or is more likely that clicking it this afternoon once or twice an hour(cuz previously it would sometimes start ok after it wouldnt) drove the gear into the nose one too many times? It was shimmed correctly and the bolts still torqued to spec.

Any ideas? I rebuilt this engine myself a couple years back, so I'm not forgetting anything on it.
I know, I know, the whole "fully charged battery" lecture aside (honestly, I was trying to start it to get a new battery!), I just need an idea as to what went wrong before I replace the nose, only to have it happen again.