The secrets to changing spark plugs on an LT1. Not that hard, even 2 and 4. All eight can be done in under 2 hours, much less with practice.
Plugs 2,4: remove spark plug wire holder from cylinder head. Might help to remove middle inner fender panel as well and go at the plugs through the wheel well, but I can get to both from the conventional route w/o trouble.
Plug 6: not very difficult, and requires nothing be removed from car.
Plug 8: will help to remove plug wire holder from head to get this, but I have done it without removing anything from the car except the wire boot from the plug itself.
Passenger side should take 30 minutes.
Plugs 1,3: remove inner fender panel from wheel well. Go at plugs from wheel well with a 12" extension on your wrench. Should take about 40 minutes. Be careful!
Plugs 5,7: here is where the real fun is. You must partially remove the AIR system. Locate the T-fitting in the AIR system just above the exhaust manifold. Disconnect the rubber hose going forward out of this T to the metal pipe that goes into the manifold. Next, follow the hose out of the bottom of the T. Disconnect it at the fitting under the ASR box. Now, the AIR hosing will lift out of your way. Take off the wire holder from the cylinder head. You will now be able to change these two plugs. Neither needs to be accessed from below.
To change these plugs you must be patient and be able to thread plugs w/o being able to see what you are doing, just feeling what you are doing. Use anti-seize compound. I use a spark plug socket with a foam insert to protect the plug and with a 11/16" hex pattern on the outside. Then, instead of putting my breaker bar or ratchet into the plug socket, I put a 11/16" 12-point on the ratchet, and use that socket around the plug socket. This is of immense help. I hand tighten as far as I possibly can, then slowly wrench tigten.