there is a weatherstrip at the firewall, just above the plenium that you can remove that may give you just enough clearance for your spark plug socket.
The metal strip that the weatherstrip is on can also either be: bent upwards with a pair of water pump pliers; or you can do what a lot of 3.4 owners do, take a cutoff wheel or a die grinder and notch the sheet metal above the rear cylinders.
guaranteed to give you enough clearance.