I have a Sentra, not registered in CA, It has a CA front converter on it. Can I replace wth non-CA spec?

Year of this sentra? 2000 to 2004 1.8 liter Sentra's have three levels of emissions standards Low emission (48 state) California emissions and ULEV ultra low emission. ULEV cars drive thru the air and clean it. Almost no harmful gasses other than co2. Your problem is going to be check engine lights because oxygen sensors 4 of them have to send signals to the ecm as if the proper cat is still on there. My best reccomendation is keep it stock save money buy that 350z you really want.

In theory yes ( if the parts bolt on the same) , but you might violate emission laws. The two parts may not be interchangeible. Usually a california emission converter is slightly different to elimintate that option. Also, Oxygen sensors are different depending on if it was a Federal emission or CA Emission requirement, the location of them may vary also.

Just put on the right one for your car and you won't have any problems.