Finally got around to pulling the intake manifold, lo and behold it is like my last guess was... Sealing surface of the intake on the bottom doesn't extend back far enough to interface with the block. You can see in the pic how the silicone isn't squished down in the corner like it should be. Looks like I'll have to build up that area myself with some JB-Weld. I guess I got an "early" casting RPM intake.
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