The heads are C5ae-f, which I believe are 427 medium risers. I thought maybe it could be a 406 block that is why I posted it here . That cast "HP" is what is really throwing me. I have not seen that before (granted I haven't seen tons of FE blocks but I have seen enough for that to make me go hmmm..) It may be perfectly normal, I just don't know.
I am currently trying to find out what the bore is and if it has been opened up at all.
What fun this is!