THANKS for all your replies - anyhow I did not notice new replies for some days.
Here are some - maybe better viewing angles:
@ Schluchtenfl.: Itīs been last that Iīve been on that. It was for sure a size that I don not own and therefore borrowed.
@ unreng: I do not own such a breaker bar - 100 bugs is over my budget for such a thing. But something similar could work out through the coil windings ... Going through the top is probably not working.
@ SteelheadJones: Penetrating stuff was used before of course, several times, several days.
Problem here is probably the following: There is some serious antirust coating all over the bottom of my truck (which is good). The bold is screwed into a welded nut on the Quigley mount - afaik. Due to existing gravity (:-)) and antirust wachs this is NOT creeping up through the nur inside/longside the bold. From above this stuff donīt get to the correct spot I guess...
I assume these are the parts - that we see in my pictures:
https://www.rockauto.com/de/moreinfo...117886&jsn=528
... which means for me I can likely reuse the 2nd thing, the rubber looks like nothing too special as used in all series.
I think a beaker bar like above + heat will work.