Check for intake gasket leaks. They are super common on FE engines. The angles of the gasket surface between the heads and the manifold need to be within .010 in order to seal.
  1. Also check the "tubes" that the pushrods run through. I've see porosities through those that introduced oil right into the runners.
  2. Remove the rockers & valvetrain.
  3. Make certain you've disconnected & plugged all vaccuum hoses or you'll pop the diaphragms.
  4. Cobble up a carb plate for the top that'll let you put a couple psi (not much!) of air pressure into the intake with a radiator pressure tester.
  5. Brake Cleaner and a strong/small flashlight...