I've used a waster plate, which is to clamp a piece of mild steel on top of the cast iron and cut through both. Same concept as Thomas' suggestion but with the added aggravation associated with stack cutting.
At work we had a pile of 2 inch thick cast iron drain grates that needed to be narrowed an inch (about 40 feet of cut) which we sent off to our metal supplier to cut as they have the biggest, baddest plasma cutter around these parts. When in doubt, farm it out!
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