They should be interchangeable as far as I know. If you want a definitive answer, contact SPL in Texas
http://splparts.com/contact.html These guys have the best suspension pieces in the industry for S and Z chassis cars.
Just out of curiosity, why are you looking at the interchangeability? The S13 arms will bolt to a Z upright. The only thing you need to swap out is the strut since the Z and S use different mounting styles.