I've tried a lot of different methods over the years to break loose the EGR fitting. A 3/4" Flare nut wrench fits close enough if I recall. You pretty much have to start with one though, or the damage to the nut will be prevent it from working after other wrenches fail. Here are some of the tricks I've tried with varying degrees of success:

I start by just spraying some PB blaster or other pentrating oil on it and giving it a try before I take the motor apart at all. If it doesn't go, I heat up the exhaust manifold and give it another go. I also tried heating the manifold up real hot either with a torch or by running the motor, and then putting ice cubes on the nut. The theory being hot manifold expands and cold nut contracts, it didn't work but the theory seems sound.

If I don't get it pretty quick, I give up try the other side briefly. On carb 2.3L motors it is very hard to reach, and on FI motors it is probably harder, so that usually doesn't work. Failing that, pull the head manifolds and all, then work on it out of the rig. Once you get one end free, you can slip a box end wrench over the free end nut and around the pipe to the stuck one. Apply heat, use a hammer and/or cheater bar on the wrench and it should let go.

I still have one that I had to cut off and never did get free. I had a spare manifold and pipe so I just swapped both parts. The other half dozen or so times I've fought this problem I eventually got the thing freed up.