This is where you really need to take everything to a competent LBS. The head tube probably just needs light reaming, and they would have the tools for that. Don't risk damaging the frame just to save a few bucks.

FWIW, I am running 700c wheels on two different frames that were originally equipped with 27" wheels. One has good old center pull cantilevers which have enough adjustment, the other required installation of long reach calipers.