2007 QX56 Exhaust Manifold issue

I am new to the forum but have owned my QX for about 2 years and absolutely love it. I am having an issue with both Cat Convertors coding I think 0420 and 0430. I believe is operating below efficiency. Is there anything that can be done other than replacement? I believe that the right manifold may also have a small crack since when cold I hear a tick. If I do need to replace, I can do it myself. What is the best way to go Headers or Manifolds. Is there a manufacturer or package that is recommended?

Thanks in advance.

Scott