Burnt oil making coke and carbon sticking to the cylinders twocents scope It should scrape off with a sharp blade wrench scope
I would take a thin bladed screw driver to try to see if all the rings on every piston are loose or not, bring it up to TDC and check both pistons at TDC to see if you can rock the pistons in the bores by pushing on top of the outside edges of the pistons directly above the rings out near the edge of the block and then again on the inside of the piston near the cam, if the pistons move freely in the bores your good, if they don't take them out to find out why they won't rock in the bores easily scope
Let us know what you find, how did the leak down and compression tests go?

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)