What kind of rear main seal do you have? Is it the rope seal? Those are known to leak pretty easily. Be sure your crankcase has an inlet and an outlet to ensure proper breathing. Your intake should have a large tube in front for the oil fill, correct. Make sure the cap is not plugged with trash. Is there also an elbow with a small tube at the rear of the intake? Been a long time since I thought about an old Chevy small block, perhaps there are others that can give better advice. I learned everything about a small block Chevy from a book called "How to rebuild your small block Chevy". Should be able to find a copy at the library, as it is a very popular book. Ken