I had a leak at the back of the motor one time many years ago (oil on the floor) and was thinkin' it was the rear main seal. After cleaning up the mess and running it while paying attention, I realized that the oil was coming from around the black rubber seal between the bottom of the manifold and the top of the rear china wall. After talking to some buddies, I learned that they didn't use those cheezy, fosdick seals any more. They just squirted a fat bead of silicone across the china wall front and rear and sat the intake manifold down on that. I've been doing it that way ever since.