I recently changed the spark plugs and wires on the 2003 Z-66 I recently bought.
The previous owner had some off brand plugs and wires installed somewhere in the past.
I had a struggled getting those cheap wires off and after the third one came apart, I ran to the parts store and picked up a box of new ACDelco wires.
After that, I took whatever measures available to get those old wires off.
Many did not survive the effort.
The spark plugs resisted removal, as well.
One thing I can say is get as good a straight line shot at the spark plug as you can and use as few extensions and swivels as you can manage.
Then, provide slow even pressure until you feel the threads break loose.
No need to heat the engine and do not give them any ugga-duggas.
Slow and steady will win the race.
The #8 spark plug (passenger side rear) is usually the one people complain most about.
I found I could slip the spark plug socket over the installed plug and then use a short socket that fit on the end of the spark plug socket and use a simple ratchet with the socket.
This works to remove and install.
I just stand along the right front wheel and reach in.
No need to take wheels off to go through the inner wheel liner.
When you are installing your new ACDelco spark plugs (notice I keep saying ACDelco), apply a tiny dab of anti-seize to the plug threads.
Don't gob it on.
A little dab will do ya.
Also, apply a dab of dielectric silicone grease to each end of the new spark plug wires.
These measures will make this job go a lot easier next time.
Best of luck with your repairs.