Above are the LN3 intake-to-head gaskets on top of the L67'*. They do line up properly. We concluded that the extra, round port is the integrated PCV. Hard to explain, but I made my own PCV system.
I couldn't figure out the leak myself. The intake manifold and */C, unless they were worked on at some point, came bolted together from the factory and there shouldn't be any conflict.
I do understand the differences between the LN3 and L67 internals. Long term durability will not be the same. But, 3800s are good motors, and mine'* pretty darn tight still. I've discussed it with the turbo guy (doing 260-280 hp out of an LN3) and he hasn't had issues with his engine or transaxle, yet. He does have 125k miles on his car and has had the setup since about 100k.
Swapping the engine would have been better, but at least on paper, would be a lot more intensive. 4T60e or 4T60e-HD is a much better choice over my existing 4T60(h) but then you get into PCMs and stuff...another, worse mess in itself. I won't push it too hard and have a really good local tranny mechanic.
I'm using the LN3 heads. Harmonic balancer, injectors, fuel rail, crankshaft PS, MAF, IAC, throttle body, */C (obviously), intake manifold, tensioner assembly, and some stuff I can't think of at the moment are from the L67 and bolted to the LN3.
I was also wondering about the intake manifold-to-head gaskets leaking, but not possible when I can see and feel coolant leaking out of the manifold-to-*/C gasket. The head/manifold gasket and areas are dry. Used a new gasket there and torqued to spec (8 ft/lbs).
Yes, paper gasket used between the */C and throttle body.[/img]