There is a big difference between the 3.1 and the 3.8....yes, the 3.1 was known for lower intake gaskets, but not the 3.8.....not saying you can't have a problem with the 3.8 lower intake gasket, but it was not as prevalent as the 3.1.....
I would be leaning towards a head gasket also.......you can do a coolant pressure test on a warm engine, and then the next morning, remove the plugs and disable the fuel and spark......have someone crank the vehicle....observe if coolant comes out of one of the spark plugs holes....if it does, you have found the suspect cylinder, and that is usually a head gasket....