I have an 05 g35 coupe and I bought new TP's for it, are the studs from the cats or the headers? I'm just thinking of cutting them off. I have all new hardware for it.

 

Soak it in a penetrating fluid for a couple days, heat it with a torch and try to turn it.

 

Give all those bolts a shot of WD40 or something like it at night, should break loose the next morning with a large breaker bar! You have the option of having a Muffler Shop swap 'em out for you! Save your cats, you may need them down the road if you ever intend to sell your G.
Gary