lost oil pressure

I don't know why the cam bearing would work itself out of it's bore but if it did that could cause your problem. The cam is the last to oil in the block before the oil is sent to the heads. The oil pressure sender is on top also. The cam bearing is probably blocking off the oil to the heads and the top of the block that's why the upper end clatters and your oil pressure gauge reads low. I would pull the engine and take it apart to check for damage. Pay close attention to how the cam bearings fit and make sure the plug down in the distributor hole is installed properly and tight. If that plug wasn't replaced after the block was cooked (boiled before machining) you could have low oil pressure problems also. Good luck and keep us posted. Toolmanmike