if u grind them. ur cat wont be useable anymore. the bolts are welded on to the cat and the nuts go in after the catback pipe. so when u grin them the bolts will come out short.

ur better off getting them off w/ in impact.

one thing i'm not understanding is that u just need a new muffler. why not just go to a muffler shop and have them cut out ur old one and weld a new one on. are u installing a catback system?