That's the same style intake gasket that causes problems on the true vortec engines. They did use a non-vortec cylinder head that used the center hold-down screws on the valve cover.
I'd first suspect this intake gasket because they are known to be problems. Next, I'd CAREFULLY inspect the head for cracks around the valve seats and pockets. As you found, a head gasket problem is unlikely. Cranking compression of 180# won't show up a possible leak, but 2000# of pressure when the plug fires will. However, you appear to have water at maybe 15# leaking INTO the cylinder. This can only come from water entering on the intake section of the engine. A white valve can also be caused by running lean.
The "bubbling" is an interesting aspect. I really suspect it is unrelated to an engine compression leak, though. They make test kits to determine the presence of CO (carbon monoxide) in the coolant. Since CO only comes from a by-product of combustion, this is a good test.
Keep me posted............
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