Those codes are set by o2 sensors. Its possible they could be at fault. All its saying is the reading from up stream before cat converter to down stream past the cat is different. Many things can cause this. Alot of people will cut into the cat itself and remove the guts if you dont have to pass emissions where you are thats the easy fix. More than likely the cat its self is clogged or restricted and thats causing your codes to be thrown. 305k is alot of miles and i would expect you to be on your 4th or 5th set of spark plugs and atleast 2nd maybe 3rd set of o2 sensors. Among many other things.