You could try backing off the adjustment of all of the intake valves so they are closed. Remove the carb and bolt on a plate to seal the manifold. Remove all vacuum hoses from the manifold and plug all fittings except one(say brake booster port) and connect a hose adapter to an air compressor and listen for air leakage?
Sometimes the rocker studs need sealer on the threads to keep oil from being sucked into the ports.
It really sounds like you need to have the sides of the intake milled to drop it down on the mating surface of the heads. By decking the block that much the bottom of the intake ports are probably too high on the head ports, not giving enough area for the gasket to seal well, letting oil get sucked in from the lifter valley. Engineczar asked how big the gap at the ends of the manifold were(front and back). I bet its lower than normal and will probably need milling as well. A good machine shop should be able to tell how much to mill depending on how much was take off the deck of the block. If you do a search, there are recomendations for the correct amounts to be removed from the sides/bottom of the manifold per amount removed from head/block decks.
Happens all the time.