It's going to be cheaper to check the fuel rail pressure than to replace the O2 sensors, although new O2 sensors on a properly running engine that you plan to keep awhile won't necessarily be a waste of money in the long run. However, if your engine is not running correctly, putting new O2 sensors in now could damage the new sensors prematurely.
Why not start with the fuel pressure since incorrect fuel pressure at the fuel rail could cause the O2 sensors to report out-of-specification readings? Do you have a list of the specific code numbers that are present?
Also, how did you clean out the converter? I'm not aware of any method for that other than hollowing one out.
-Rod