Bolts missing --> Cracked 904 case

My guess is that someone didn't get the converter seated all the way into the front pump and pulled it in with the bolts. Possibly leaving a gap between bell housing and block. Once the crack progressed it relieved the load on the bolts and they vibrated out. I personally would use a torque wrench and torque things to spec. Maybe use some on the flex plate to crank and converter bolts.