700C is about 4mm smaller than 27 wheels. My bike is a conversion of a schwinn frame designed for 27wheels, I put 700's on them and it's fine. Unfortunately my frame is drilled for nutted brakes and my LBS only stocked the Tektro 538, which barely reached the front rim (enough to brake, though). I ended up filing about ~2mm from the fork which brought the brakepads to a more comfortable part of the machined surface of my rims and all is good.