I think I'd try a sawzall with a sheet metal blade (lots of teeth per inch). There will probably be some spots you won't be able to get to with that, but you could switch to a cutoff wheel on a die grinder for (possibly) improved access. That would cause lots of sparks, but unless you're soaked with gasoline, there shouldn't be a problem with fire. But in terms of safety, always have a fire extinguisher handy. And if you're cutting from underneath, be sure to wear a full face shield.
[shoot, Martin beat me to it
]
