I was a professional mechanic for over two decades.
In that time I have seen more than a few stubborn exhaust fasteners.
Some of the worst ones only responded to a combination of penetrating
lubricant and carefully applied percussion waves generated by an pneumatic
hammer with a small drift tool attached.
The force being applied at a shallow angle to one of the faces of the nut.
If the threads are not too badly fretted, the nut should spin off easily
after being "broken loose" with the hammer/drift tool. Worst case scenerio,
you can torch them off, drive out the remainder of the studs and
"through-bolt" them when you reinstall.
Best of Luck
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