We recently had a similar problem with a 427.
Make sure:
-the pins that hold the rope seal in are out if you use a modern seal.
-the three block plugs on the back of the block are in and tight.
-the PCV system is operational.
-the seal is in the right way.
-stagger the ends in the main cap.
-silicone the crap out of it and the side seals.
-the oil pan isn't the real problem
Mike