Thank you all for your tips.
I got all non-cooperative screw's out with various tricks.
I had my battery charger beside the truck since I had tried to start the engine with a 6V battery that doesn't hold charge anymore and got thinking. One (or several) of you mentioned heat and I tried something new for me. I connected ground to the ground cable and cleaned the head of my first 'patient' up clean & shiny and held the positive from the charger still on 6V on to the head. I thing I wanted to avoid was fusing the threads together, so I tried this with no change to the willingness to turn. Next was 12V 30amp for probably 10 seconds and this screw came with my impact M18 impact driver with several gentle reverse & forward actions.
The next screw was more stubborn and did come, so I tried 60amp for 10 seconds and this one came too. I don't know if it was truly the heating or just luck, but something to try again if needed.
