That is a Mega block, not a OEM Mopar hemi cast block the cast in M being the key I look at that picture a little closer and maybe that is NOT a M I have a Mega Hemi block motor in my shop now in a 1969 R.R. so I will look at it a little closer later, it definitely has the cast in M on the passenger side, I'll have to look at the casting number on that side of the block
Cab here are the 2 mega block castings 1 has full water jackets the 1 with Mopar is siamese.