I've had varying success with them - they've always been at the 'budget' end of the quality and can't say I've used them for many years.
Personally I would find a socket that fits - it may well be an imperial fitting depending on the age of vehicle.
If you're on your own I would use a molegrip on the nut on the underside, and a vertical screw into the wood to stop the grips spinning, then a socket and extension on the top.
I accept I'm a bit over cautious when it comes to grinders in the vicinity of the fuel tank, but as you're underneath, and replacing the wood, I can't see any issue with just cutting through the nuts/bolts from the underside.
