What engine do I have?

So I'm trying to figure out exactly what I have in the car.  According to the sticker and build sheet it should be a 400, but I can't seem to find the code on the block...  Best I can find is the Y4 shown below, but that doesn't correspond to any engine in the '77s I can find online.  I guess not knowing the history of the car is one of those endearing quirks of vehicles that have been played with for 30 years...