PB Blaster should work. Spray the piss out of it. If its really bad, spray it everyday for a week or so. This is really gonna be good if you drive it daily as well.
Then, you can use a torch to get it out. A propane torch should get the job done. If is really bad and you have access to one, an Oxyacetylene torch will work, but a bit overkill lol. Also, when you try and get it out, do it when the engine hasnt been driven for a while. (like a day or so). Lastly, use a big ass breaker bar to get better leverage. Hopefully the one you are trying to get out has enough room around it to use a nice sized one.
There are two on each side of the car, one under each exhaust manifold beside the engine block, feel up behind that, it is easy to find em, then two under the car, after the precats sitting on top, it isn't hard to fit a 36'' breaker bar up there if it has a swivel end on it. But in the engine bay, yes it is nearly impossible to fit one that big, but you can do it from the bottom, you just won't get a large turn radius with it.
This is on the 3.3, I am unsure of how hard it would be to get to the ones on a 3.5....