i dont think that a full system for a 300 would be able to fit under the infinity, unless the underbody is the same, or to where the exhaust will fit without hitting anything or melting anything. im not really sure tho.
the length of the infinity is another thing to look at compared to the 300.

 

If you can weld and have access to mandrel bends, you're better off getting a bunch of piping and a Greddy or HKS muffler and going custom. Just get the car on a lift and map out a planned route and then weld in the pipe, hangers, and muffler...and cat if he wants it. and youre good.

 

the J30 just has the same block. that's about it.
the exhaust isn't dual like the Z.
Go to (it hurts to say this) www.stillen.com and look there for an exhaust.

 

No it wont fit (and yes I know this for a fact I own a J30 and have been under it many times). The J30 has dual exhaust until after the cats back then goes back into single, just about the only option would be to have a custom system made.

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