Getting damaged header attaching bolts out falls into two basic categories- - - -difficult and impossible. Since you're planning to replace the whole system anyway, take a sawzall to the header tubes, cut them as close to the heads as possible, and remove the heads. Once they're on a workbench, use the sawzall, a torch, a grinder, or other types of cutting tools to remove the bolt heads and the flanges, then use a torch and vise grips to get the bolt stubs out of the heads.
A word to the wise- - - -when you replace the exhaust system, seriously consider what caused all the grief to start with- - - - -headers! Your situation is a silent, eloquent argument for running cast iron exhaust manifolds!
Jerry
