Since you are replacing the ball joint anyway and wouldn’t care of the assembly gets hot, why not as you said spray her down with a half gallon of penetrating oil, let it soak and then apply as much heat as you’re comfortable with. I’ve found that if I can “expand” the metal around a frozen fastener ever so slightly.. that I have a much better chance of getting it broken free. Just a thought - and I wish you all the luck breaking that sucker loose so that you don’t have to tear the rest apart [emoji106]

Edit - I also like to take a hammer (if you can get it in there) to the head of the bolt to break some of the scale loose. You can hit the hot assembly as well and see how that goes.

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