They're definitely a mother to get off. If you're just ditching the old h-pipe, get new ones for the new h-pipe. Be sure to put antisieze on the threads so you can get them off in the future.
Is the h-pipe in or out of the car at this point? If it's out, use a 7/8" wrench with a cheater pipe.
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351W 89 Mustang GT Convertible