Is this a road bike to fixie conversion? The frame that you'll be using, was it intended for 700 wheels? Not that fitting 700 wheels on a frame intended for 27" won't work but you could increase the chances of pedal strike when taking a corner at a good clip... something that isn't much fun when riding fixed. This is what many people doing fixie conversions don't think about and it gets even riskier when using road cranks with 170mm or longer crank arms instead of track cranks.