If the cylinder walls are mirror like, they are too smooth and the rings will never seat.
That would account for the blow by.
300 oil temp without a cooler would not be too alarming.
If you are going to pull the pan, I would say go ahead and undo the bearing caps one at a time and check them.
If they are damaged, it will be obvious.
while doing that, pop each piston out and check for broken rings or ring lands. My guess is that there will not be any because of the condition of the cylinder walls.
If you can swing the $ send it to Curtis to reassemble. If not, get a friend that knows if you don't what to do and put it back together. Make sure you find the source of the clatter.
Check the valve clearances. I have had one loosen up and it sounded like the rods were coming through the block.