Whent to a performance shop in town asking for HF Cats for my G35 sedan, he told me they are all universal and suggested Magnaflow I have aftermarket headers and thought there was a specific style of HFC for the G is this true can I use a universal or should I look for one specific to my car what are your thoughts?

 

There are plenty of HFC's made specifically for the Z/G: FI, Crawford, Berk, Stillen, etc. plus plenty of jdm choices as well: Power Enterprise, Mines, etc.