If you have the manifolds off it will be easy. I did mine this way. Go to one of those surplus buy bolts by the pound places. They will most likely have a bin of tooling stuff. I bought my 19mm allen wrench for 2 bucks! Then I used a big pipe wrench (the one that one has in the corner of the garage for this very job) this thing is about 4 foot long!
Using the pipe wrench does not limit you like the big piece of pipe does on an allen, and you can get the wrench close to the plug for better power.
Good luck,
Tracy