This is a brief update. I've removed the intake manifold again in order to deal with that water leak. I did notice quite a bit of pitting at the water jacket on the head where the leak was, so it's quite possible that's why it was leaking. I will reassemble using a grey RTV sealer at the water passages once I receive my next set of gaskets. I also plan on replacing that leaking expansion plug on the block before I pour in the removed 3 1/2 gallons of engine coolant.