It seems to be a 390 and a thick one at that to live at 4.13. The 390 and 352 blocks are identical to each other pretty much. It must have been a factory screwup of some kind. The whole piston deal is still kind of odd though but sounds like you have a great start with the block. The thick 390's are hard to find for sure. Good luck and keep us posted how thing progress. G.