When you say ratchet, what size of drive ratchet are you trying? You need to hit that with a 3/8ths drive air impact turned down to a low torque. It makes all the difference on those stubborn threads. I bet Madbuyer's right about the thread locker. Look for crusty white on the threads when you do get it out.
If air is out of the question, use a pipe cheater on your ratchet - carefully right I know you know. If you can give it a bit of a sharp effort, after it's had a chance to soak up some of that Pb shixit. I love that stuff, but it reeks!