You'll prolly find that your brake pads just hit your tyres by a couple of mm even when they're moved all the way down... in order to avoid fitting longer brakes, you can just grind/file a chamfer along the top of your pads. Concrete or asphalt is usually pretty good for this.
You may find your front brake takes more of a squeeze than you'd like, thanks to the reduced leverage... you'll have to fit a bigger brake (not just with longer pad mounts) or a 700C fork to fix this.