The thing that bothers me is the location of the swipe mark on the flex plate. If I'm thinking correctly the only thing that protrudes off the back of the block in that area is the camshaft plug. With what you described I would have to say the plate is deflecting as the rpm's go up from a very worn fit or broken flex plate.

I've seen this happen on electric motors with plastic fan that don't rub turning by hand but hit the endbell or cover when running at full speed.

It will be interesting to see the pics of the damage