Well, I pulled the intake and pan off to hunt down the mystery knock, and with the help of John D., we might have found the culprit.
Rod bearing #1 on the left, #5 on the right. This is only 250 miles. Looks like we have a brand new block that wasn't cleaned properly.
I'm very lucky something didn't go REALLY BAD!!!
I'm requesting the T.I.T. for next Sunday to pull the new 427......damn.