340 with metal in oil pan

Tear it down. That's gotta be a valve lock, don't see how a seat could end up in your oil pan anyways. Plan on going through the whole thing as long as it's out; none of it looks promising- but at least you didn't ventilate the block. You might be able to save the rotating assembly, just inspect EVERYTHING and replace what's necessary. Then reassemble it RIGHT. You've seen the results of shortcuts.