Thanks Stu, I still can't get a concrete answer, I went by my local futureshop and was told I needed an install kit to install a single din deck, when I said I was installing a double din the sales person couldn't tell me and the install guy wasn't in. 

I measured the face of the new Clarion CX400 deck and it's shorter and narrower than the stock nissan 6 disc plus cassette face by a full quarter inch.  Is that what a mounting kit is for to fill in the gaps on the face?