How exactly do you "free up the exhaust"?
First thing to do would be to remove the resonator (It's the "muffler before the muffler"). Best way to do this would be to remove it and weld a straight pipe in place. Make sure you secure it though so it doesn't bounce around!
Second thing to do would be to get a high flow muffler, like a Dynomax or Magnaflow :p