problem with slip on's are that there fibreglass and there never 100% ... i know as i have a set sitting at home, it doesnt line up with the stock gtr side skirts as there a 2 piece side skirt. the add ons are actually made to fit the N1 side skirts as they are a 3 piece side skirt... the add on slips over the middle piece
if you really want to use it you will have to chop ur origional skirts as the slip on is the same width as ur skirts
if u manage to find a set of origional N1 side skirts .. dummy fit before you paint it (if color is different), all the factory side skirts warp over the years so u will find fitment is never really spot on.
the N1 origional skirts i found ended up being warped... nothing stickerflew wont fix but there never coming off in a hurry...
good luck in your search