In your first photo of the bare block, look at that area between the first two main cap crossbolts on the right hand side of the block (directly down from the X cat in the block and 1 inch up from where the oil pan attaches). That's where the VIN will be stamped if there is one. I've found quite a few blocks out of 71 Hemi cars that were cast in 68 so you might be able to make someone very happy and pocket enough pesos to complete a Hemi build.
If it isn't someone's missing block it looks like a great builder anyway. Personally I would throw away the KB pistons and put quality pistons in it along with a 4.15 crank.
Sheldon