Newb to the forum, and relatively new to Nissans, but am a regular in the VW/Audi community.
Sounds like an exhaust leak pre-cat. If you know the manifold is cracked, then you found your leak. Replace that or plug the leak; (JB weld would be good enough for a temporary fix) put a new gasket on, clear the codes and see if the code comes back. If it does, you have it narrowed to an 02 sensor or a clogged cat.
The code quotes "below efficiency," which means the sensor is not reading optimum flow-through, or the gas coming out is still dirty.
If your Cat needs replacing, you need to ask why. As long as it isn't rusted out they should last the life of the vehicle. A bad cat is a symptom of other issues.
Just replacing the cat is like using nasal spray to clear your nose. You'll breathe easier for a little while, but you're still sick, and the snot will be back.