Use a box wrench with a cheater pipe and just apply force to it slowly. Once it cracks loose it should be pretty easy to remove. Use a penetrating oil like Liquid Wrench, not WD-40.
Heating the exhaust pipe is a good idea, but may be hard to get your hands in there without getting burnt. I don't know how much room you have to work, though.
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351W 89 Mustang GT Convertible