02 exhaust help please :(

No .. neither of those codes have anything to do with exhaust leaks. The 0455 code is related to a large leak in the evaporative emissions system ( fuel vapor system etc) . And the P0457 is about the gas cap being loose or off. the exhaust wouldn't trigger any of this. Once you fix the gas cap problem, those 2 codes will go away, though they still may need to be cleared from the system.

And chances are your exhaust leak is a small hairline crack in the passenger side exhaust header/manifold pipe. The Exhaust gaskets RARELY start leaking, and I have already heard of a few dozen instances of other 99-04 3.8 Mustang owners having the same problem. Headers got a defect in them. But I dont think Ford ever recalled it. A good way to find an exhaust leak, take an old rag and get it soaking wet. Fold the rag a couple times.. start the engine and carefully ( while not burning yourself) take the rag and move it around to different spots on the header pipe. it the noise goes away when you touch a spot, you found the leak. .... that's just my little idea of finding a leak. Maybe there are better ways..