Yup, it's the air injection tube. #6 is where it usually rusts through. Standing in front of the truck, looking at the engine, right on the passenger side of the valve cover, you'll see this metal tube. It branches into each cylinder at the head (see pic). The whole thing is gonna have to be replaced. Pull the upper intake off, and inspect it. It is possible that it's an exhaust gasket, but you'll have to take the intake off for that anyway, so check this first.
Take care,
~Chris
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