Here's my $0.02, oxygen lances work great. We use a lance system that's essentially a thin walled (I think aluminum) pipe, with magnesium rods in it. Heat the working end up with an acetylene torch, then open up the O2. Slices through just about anything.
Also, in our scrap center "burn shed" (guess why we call it that), we use an oxygen lance system that feeds "scarfing powder" to the torch tip. Very dazzling at dusk or dawn. We cut solid bars and bundles of stainless, tool steel, etc., up to around 15" in diameter to make a more manageable scrap charge in the arc furnace. I'm sure this system would be impractical for your one-time job, but I figured I'd let the community know about some lancing/scarfing capabilities.
Both systems rely on heating the metal (lance pipe or powder) to it's melting point, and then adding O2 to essentially start a molten metal fire, which pumps out enough heat to melt through whatever you're cutting.
In any case, both methods produce a LOT of heavy smoke, sparks, and some molten metal spray, which is directed away from the "burner" by large fans. In our burn shed, the burner also wears a helmet with a powered HEPA filtration system.
I agree with CWIC, that renting the system would probably be the most cost effective way to go.
And as long as we're having fun, and if you don't want to go with a lance, why not chill it down with some liquid N2, so it gets nice and brittle, and then just hit it with a big hammer? Or an ACME wrecking ball.