Sometimes, an over filled crankcase increases oil pressure inside the engine block. Have you checked the oil pressure? This check is done near the oil filter housing. There is a fitting that you simply unscrew, screw in a generic oil pressure hose, and read the PSI at idle. The higher the rpm, the higher the pressure. This will eventually "blow off" to a specific number (depending on health of bearings, ect.). If the pressure keep rising, and does not stay steady when increasing rpm, you have a stuck bleed off valve. This might be your problem?
Now for the manifold. You might have a poor fitting intake manifold. The leak you think might be coming from the back of the engine, just might be the oil leaking out the back of the intake manifold. Install the manifold "dry" in place. Use 4 manifold bolts and tighten the 4 corners. Do not use any silicone...this is a gap test you are going after. Take a mirror and inspect the gap between the surface of the engine block and manifold. If the gap is anything larger than 3/8" of an inch, you may have to machine the maniflold and get it to drop a few 1/16th of an inch.
Does the ball in the PCV rattle? This also may increase crankcase pressure.
The best silicone to use for the intake manifold is sold at GM. Tell parts you are installing the intake manifold and he will hand you a tube of the good stuff.