another overheating problem

There's a kit you can borrow from the parts store that adds dye to the coolant. It reacts and turns a different color if combustion gasses are present. You should get a straight edge and check the block deck, too. I don't know where the .381" figure came from, but the go/no go spec is .002". If you can get more than a .002" feeler gauge under a straight edge on the deck surface, it needs truing up.

The proper way to check it is parallel with the crank front to back, then at opposite opposing corners. The blocks are notoriously known for being high and "twisted" at the deck surface right from the factory.

Good luck with it. Keep us posted. I hope you get it fixed.