I suggest:
1/ PATIENCE!
2/ If the pole piece is rusty use an acid rust cleaner or vinegar then foaming cleaner then alcohol to really clean it out.
3/ use epoxy to glue an allen key into th hole. Leave it fro 24 hours in a war place. Then leave it for anothed 12 because you don't beleve me -no-one does. They "test" the joint to see if its nearly hard yet. Chemical bonds are permanently damaged if you move epoxy before it is fully set - 24 hours got that?
4/ soak the thread in penitrating fluid for 4 hours.
5/ Try it.
Quick method. Drill a deaper hole an use a screw extractor. Probably won't work.
Best method.
Save all of than time and expence and get what you really wanted anyway. A brand new hot pickup
