pipe wrench ?! on threads? From the top, lube with a good penetrating
oil, allow to soak for a week. Heat is helpful but the proximity to the body cancels that. You may remove shocks, drop the axle as far as possible, with out damaging brake line,! and then remove the coil spring, exposing the bottom of the adjustment thread. Yes it is a pita to replace the screw assembly. If you let it soak and get the adjustment screw/bolt to back out the top, then run a tap through to clean up the threads then replace with Stainless bolt. Worse comes to worst you will have to drill the bolt out from the bottom and with a easy out (the easy out in this case would be for a left hand thread!) remove the bolt. It will require a big bit and hopefully a magnetic base drill with very careful alignment . Remember a 1/2 drill with a bit approaching 1 inch can be a wrist breaker !! OR -- perhaps a couple of spring spacers would give you the clearance you desire.