OK...that woud exclude the intake manifold gasket as a cause.....! THere has been a lot said in the "timguili" thread that you probably have seen....
I do not have photos of the 3.4 but just as the other blocks, thermostat is normally easy to get at, and there should be a plug on the thermostat housing that can be revoved to vent air from the system.
Changing head gasketS is an awful job given the position of the engine but is listed as an "on vehicle service"......I've never done it on this vehicle so others should come and list their own perception of the job. Exhaust crossover pipe, exhaust manifolds must be removed , the a/c compressor too, etc....not a very "user friendly environment......
I have the detailed instructions for the 3.1 and 3.8 and they are much alike and are surely a very close indication of same steps with the 3.4
If need be, I'll scan them for you unless someone comes with the exact story for the 3.4