well the pics are not much help....will need the casting date which you didn't take a pic of, above the
oil filter housing. and the side of the block...to show if it has the side oiler galley.....
however i will say the "I" or "1" on the back of the block is usually on marine or industrial blocks...so it looks like that's the way its going......so far my bets are on a marine block.
the 66-427 is on most FE's post 1966 as it is the core that was used in the foundry for all of them. it appears you have a galley in the back so it looks as though it is in fact a hydraulic block