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If not already, use a steel wire wheel on a drill or a steel wire brush to remove rust formed on the exposed stud thread.

Use a coarse and then at the interface of the nut where the stud thread erupts, a fine wire wheel to gain some relief at the mound of corrosion of the rust where the nut inside diameter begins at a usually slightly chamfered edge on the inside diameter of the nut. Wear eye protection. Maybe breathing protection for the rust that become air borne powder.

Doing so may help penetrant oil to enter the thread interface between the retaining nut and the stud.

Also, as Haynes says it in UK commonwealth English, firmly, yet gently tap around the top of nut using not a small hammer, maybe a heavy screw driver handle, and pointed punch to at least cause vibration to foment fracture to the corrosion bond between the nut and stud. The vibration may work into the carbon-steel stud to light aluminum alloy cylinder head.

You can also use penetrant like the Liquid Wrench, Kroil (see gunsmiths that sell it), and PB B'laster (you are already using) Penetrant. I'm unsure which penentrant oil is best for weakening or dissolving iron alloy rust corrosion on factener hardware any more.

In articles here on your brickboard, there exist some notion of a 50/50 mix of ATF (probably Dexron, Mercon, and the like, not "Type F or G") and acetone to loosen fastener hardware yet the effect does not survive heat and does not attack corrosion.

Hope that helps.
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