2012 GMC Terrain 3.0L SLE VIN 5 (Flex) 200k miles I assume that I really need/have to remove the intake manifold when replacing the (rear) spark plugs. Which I would assume means replacing the gasket(s). Would that mean replacing both the upper & lower gaskets? Throttle body gasket as well while doing this job to clean it? Using iridium plugs AC Delco 41-109. First time for me to work on this vehicle so appreciate any insight. Thank you.