the stock exhaust is all welded, so to put new mufflers on you would need a shop to cut the old ones off and weld on the new ones, chances are the res is shot too, or close. the easy way is to buy a new cat back set up. heres some links to all the parts you need. feel free to shop the net more for better prices, they may be out there.
mufflers, walker 04 2004 Pontiac Grand Prix Muffler - Exhaust - Walker - PartsGeek
intermediate pipe 04 2004 Pontiac Grand Prix Exhaust Pipe - Exhaust - Dynomax | Drivewire.com
and a res, just so you know how much to piece it together. 04 2004 Pontiac Grand Prix Exhaust Resonator - Exhaust - Walker - PartsGeek
after you break the bolts from the res to the cat, its only hangers holding it up, take it off as one unit.
when you have your new set up, put it together in your driveway, and then slide it under, hangers first, then bolt the res to cat, then align everything so it looks high and tight, then clamp it all down tight.
dont forget to get a gasket for the res to cat flange. and clean the the cat's flange up good.
i backed my car up on ramps, and then jacked the front up and put the front on jack stands, that way the whole car is up high enough to get under it mid way to back bumper.
4 jack stands would also work, i only have 2.
