Providing the Nissan one is in good shape you can reuse it, it's good practice to swap it out for a new one, when we supply and fit we fit a new gasket...
Welcome to the forum, you can talk to us, we can help with most parts - http://www.cougarstore.co.uk/
If you need any specific advice or help, feel free to call or email us and we'll be happy to help...
Also, you can turn the clamp round so that its sits on the top...
Lots of images here - http://www.350z-uk.com/topic/63096-cobra-sports-exhaust-from-cougar-store/
Its a great car, we have a few we look after, and one of the owners is actually selling his 2002 model, which we recently fitted rear pads and discs too...