If there are Road short reach brakes on there the fork Blades are too short to be longer ..
You proposing to move the fork crown further away from the dropouts?
Perhaps more shopping is in order , a new bike for the fleet..
Spec' Tarmac (road), Ridley Fenix (commuter & fast tourer), Salsa Mamasita (MTB), CoMo Speedster (tandem), Surly Big Dummy (cargo), Airnimal (folder)
What fietsbob is trying to say is that the brakes are probably not the limiting factor here, it is the frame clearance that is the problem. The brakes should be bought to match the frame - the frame determines everything. Changing the brakes probably won't help. However, if you post a picture of the bike, with close-up on the brake area, then it will be possible to confirm this for certain.
I replaced the brakes, which gave me tons of clearance, but ultimately I was limited by the frame. The largest tire I could fit in the rear (where clearance was an issue) was a 700x30. A 300x32 with aggressive tread fit in the front, but not on the rear. You might be able to find some longer reach brakes, or go retro like me and use some old centerpull (better centerpull calipers can be had, i.e. Mafac Racers, etc)...It works just fine. I'll have to get out and ride it this weekend and see how it works. It is possible to use a road frame for CX, but ultimately it's a band-aid. I figured I'd ride mine for a season or two and see how I like it. If I totally dig it, I'll get a CX specific frameset, and if not, I'll turn it back into a road bike.