like US1 said, also check piston to block clearance, maybe it is a little to tight , good luck and let us know what happens

It still has stock pistons and Std. bore

I would say you overheated it too. I did it once when I put the raw water pump on backwards. I won't do that again. Luckily the motor wasn't hurt at all, I changed the hoses around and the next day everything was great.

check your distributor gear. If you put a billet cam in there it would have eaten your standard distributor gear and thrown the timing way out of whack. A bronze gear would then be required..just a thought from experience....

Sorry for your troubles. Just a question on end clearance and rear main install. Did you use a dead blow hammer to seat the bearing thrust surfaces when crank was installed. .006 end play was checked after all main bearings installed and torqued down? Any pictures of failed bearing and crank by chance. Machine shop had to polish or grind thrust area on crank. Did they check it for straightness? What did the other main bearings look like? Gotta make you nervous with NO clear answers why bearing failed. I know it would make me nervous Sorry for so many questions.

Check the drive coupler. Make sure the splines didn't twist. Did you have to draw the drive in the last 1/8 th of an inch?

If you got a look at the water pres guage before it "locked up", that may be able to help you (or maybe the oil temp...if it was really hot). Any clog in the water line can disrupt flow and if that impellar gets hot enough it will shut the motor down. Happened to me when my water line clogged. It about gave me a coronary! I know you said the impellar looked fine, but maybe it heated up without ripping apart. Not likely perhpas, but that squeeking/squealing sound seems like that to me. Just my .02, alotmore knowledge on this thread than I got.....

Possible bent flexplate or maybe too much grease in the coupler splines. If the coupler has too much grease it will hydrolock and put a huge amount of pressure on the thrust bearing.

sure sounds like it was getting hot. I'd bet the squeal was in fact the impellar. What about your water pickups are they all clear? Are you sure it is actually getting water?

I still like US1's question. Did you set the piston rings to "marine" gap instead of auto? Boats run hotter pistons (always under load) and cooler blocks.

Good luck.