First, welcome to CHR. I typed a reply earlier, but it didn't post and I lost it.... My first thought was that you lost a lobe on the cam, and your statement about tightening one lifter to reduce the noise points to that even more. I'd drain the oil and check for very fine metal dust in the oil, and maybe cut the filter apart and check in the folds of the filter media, too. I assume that the rebuild of the 327 did not include a conversion to roller lifters, right?

On the leak, look at the "China Wall" gasket surface across the back. They're known to blow out, which is why using a thick bead of silicone has become the preferred method of sealing vs the rubber gasket.