67, sorry to hear about the bottom end failure, but after looking at the pictures you posted, I'd put my money on a bent crank. When the converter ballooned, driving the flex plate bolts into the back of the block, the shock load was transferred through the crank. Resizing the crank to fit the 400 block may have enabled such damage.

As I look at number 4 main (and yes, I realize unmodified oiling can show accelerated wear at this journal - but rarely after just 5K miles is assembled correctly, you can also detect excessive wear in the number 4 journal - relative to the rest. I'd be willing to bet it bent the crank. I don't see anything that suggests a problem with oiling. I think it's the byproduct of a catastrophic converter failure. Just my SWAG, and I may be wrong, but I'd sure like to know how it checks out.

Southernman