There is a slight ridge, but I can't feel it by rubbing my finger over it, I'm thinking a tiny bit of scotchbrite would get rid of it, you may be onto something.
Just Great ! I would bet you Lunch that it is the High Nickel 2 bolt main race block. 318 blocks are built tough to begin with, ever compared one to a 289 ford block. The ford block looks like a toy and cracks between the compression side of the cylinder and the water jacket, pretty much junk after that.
Good 318 Block and add a little more nickel to the cast mix and you got a tough one. Always enjoy retracing the mopar engineers footsteps, those guys could hardly wait to get to work in the morning to continue building the mopars.