2003 Chevy Impala catalatic convertor

Spray the fasteners with penetrant. Remove the bolts from the upper flange at the rear bank exhaust manifold, remove the oxygen sensor, and remove the bolts at the rear flange to the exhaust pipe. That should be about it. With it being exhaust, the fasteners can be stuck in place, and can break during removal, requiring drilling and extracting, which is not fun. If you chose to take it to an exhaust shop, they would most likely cut the pipe, and weld a new one in. An aftermarket catalyst does not have near the service life as an O.E. Replacement, that explains the major difference in price.