Another option is a dremel or angle grinder to just cut off the head of the bolt or nut. Of course that is a concern next to the gas tank.
But they're not a deep-well socket, which would probably have been what you needed.
I've used these as well and they do work very effectively as well.
But, they can still twist off a bolt head.
Anytime I need to do a job on the undercarriage or exhaust, for a week prior to the job, I spray WD-40/PB-Blaster/Pentrating Oil on the nuts and bolts, once a day, every day for a week. Giving it a week to soak up the penetrating oil and allow it to penetrate the threads and loosen the seizing from corrosion, and the nuts back out like normal.
Penetrating oil just before the job, doesn't give it time to penetrate, for seriously corroded fasteners, you need to keep applying the penetrating oil for days.
The only time it hasn't worked, is on bolts so long or large, that even a week of applying penetrating oil every day still didn't get it to penetrate enough.