Block Strength

Well I had a catastrofic failure three monday's ago in the dart. Out for a little cruise to see how the pinion snubber and a new set of shoes for the back was going to react before going to the track on wednesday. Rolled through water across the road really slow and lit the tires after coming out of it, right after hitting second she popped on me. That night pulled her down to the heads and ended up taking the driver side head off. When it popped she was blowing hard out that side. After that head pressure tested out just fine we pulled the motor on out and found a broken #6 rod. That was a suspicion but we were hoping for the best. After taking the broken rod and piston out of the hole we found massive block failure at the bottom of the cylinder towards the outside. There are at least 4 vertical cracks and one that is horizontal to the cylinder. Is my logic right to assume that the block failed causing the cylinder to hydraulicly lock, breaking the rod and the dual plane intake explains the majority of water in #1 and 7? I have always been under the assumption mopars were really stout when it came to their blocks. Could the addition of a 4" crank and a .030 over bore weaken the block that much to cause that kind of damage?