Maybe I'm the only one who has ever had the head drain get loose and leak dripping off the bell housing (no one seems to know there is one because you cant see it without a mirror) but mine did leak.

Located on the back side of the head between the firewall, (you'll need a mirror) there is a dime sized hex oil plug screwed in with aluminum washer. They can, and do get loose and leak!

I have replaced both rear and front main seals. Easy to do with the engine out. Also, the IP has a large rubber O-ring between IP and block.

Also, the intake manifolds tend to leak black messy residue if not real tight, or flat. Use a straight edge to check intake manifold flushness, more often they are not perfect (need to mill flat) and the gaskets dont always compensate. Residue runs down the side of the block and then gets blown back on everything (if you dont have a body pan) and can look like motor oil. BB