I had one do the same thing about 10 years ago. Mine had Childs & Albert rods. None broke, 4 bent. Stock block with aluminum caps. Never ran the motor over 6700 rpm. It had a Mopar 4.150 crank and Venolia pistons. Heres what I think happened. During the morning warm up on jackstands I could fell a very slight vibration but couldn't find anything unusual. I was making a pass and had him coverd so I lifted. It was then that I felt a big vibration. Looked in the mirror,no smoke. I though I broke a u_joint. I coasted around the shutdown. Not until I looked under did I see the hole in the block. During disassembly I discovered the crank broke in half at the front of the #3 main. It pulled both the #1 & #2 bulkheads out of the block. The rear half of the crank and rear 4 pistons were still intact. No bearing damage at all except the #3 main thrust where the crank broke. The block had had a freeze crack in the side ever since it was built. Moroso Ceramic Seal had sealed that crack. Aparantly it had also sealed any internal water jacket cracks. I think the block broke the crank.
Doug

I agree! It also had stock caps by looking at the photos

If you want to race now. I have a 440 with a ton of good parts. I guess it makes 600 hp. It ran 8:20's in a rail. $5000.00 complete. Carb to pan everything ready to run. KB pump,jesel belt drive,roller cam, rev kit,team G intake,roller rockers,Milodon swing pan,Dominator,cap, dist, wires,JW flex plate,after market rods,pistons and damper. It also has a set of I believe 906 heads ported by a guy on a NASCAR team. This motor always ran really good, My frends wife raced it one season and lost intrest. I haven't finished putting heads or pan back on I could send pictures before I put it all back together.
Thanks Joe

It's fairly common knowledge the bb mopar will start to pull mains out. Once the support for the crank is gone the crank flexes and eventually lets go. Look for cap walk at the parting lines of the main caps. If there's metal transfer then the timer ended and the bomb went off...