A universal converter is less than $100..........but you have to weld it in.
An "Exact-Fit" type (direct bolt-in, no mods) is $50-$75 more.
A new cat won't do much for NOX, does wonders if you have high HC's.......
Also, new cat's need to be "burned-in" about 50-100 miles before they work at peak efficiency (sp?).
Good Luck
