Matching the 27" necks to kit bodies may not always give quite as good fit as the standard 'made for body #XX' paired necks. How long since you received the kit? If you've only just got it (and you're not in Perth) give it some time to acclimatise before cutting shims, as the gap may change a bit. It'll still need some filling, but better to let it settle before cutting and gluing anything in place.