I've built a couple of 3.875 strokers both a 415 and 421.

I've seen pistons broken like you described. They were in race motors that experienced a lean out or way to much spark advance which causes detonation which breaks the crown.

You mentioned 104 octane is this also a 12.5 compression or so motor?

My other question is what kind of heads are you running. Heads like my Dart 227cc only require only 32 degrees of advance to make max power.