I know we threw out a number of ideas on this trying to help you out, definitely interested to see what Paul finds.
It reminds me of a lower end problem I had years ago with a 455 motor that drove me nuts for months and it was a fresh motor. Shortly after running it, it developed a knock. After finally giving up with it in the car I pulled the motor and upon tear down found it was caused by piston slap. Needless to say cylinder issue found that wasn't addressed in machining.
It was definitely painful, but after you get through all the pain and start testing your traction issues it becomes a distant memory... Keep us updated, most all of us on here like to continue to learn!!!