[Sirrotica]

It's most likely a 77-78 as these are the only years that had only the late 3 hole motor mount bolt pattern. The year is most likely, but not always, 77-78, next to the distributor in 2 digit form there is also a longer date code cast in if the 2 digit number is absent. The 557 is probably the weakest Pontiac 400 block. Good at least for a strong street engine 450-500HP safely. Good to hear you finally found a usable block locally.