First - What month/year is your 350Z and is it Japanese spec (right-hand Drive) or right-hand drive spec for your Country?

FYI: The earlier US spec (2003/04) 350Z springs and sway-bars were softer than the later (2004.5 - 2006) model years. I do not know the spring ratings/sway-bars of the 2007-08.

Second: You may want to research having adjustable end points sway-bars to set-up the suspension for your driving preference (understear/overstear). Also the end-links are best to be replaced when changing out to an adjustable sway-bar to either "pre-load" or netural set-up for your driving conditions.

With this in mind, their are several web sites that can calcuate the sway-bars for your driving conditions. I seam to recall that SPL has a spring/sway-bar calculator (calcuated in US measures - i.e not metric) to determine the size,etc. of the sway-bar/spring rate combinations.

Good Luck.