Look at the picture at the bottom of this page:
http://www.rodbikes.com/articles/brakes.html" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;
Typically it is a very bad idea to jury rig disc brake tabs onto a fork or frame that wasn't designed to handle the additional stress from disc brakes. For cross maybe it's ok because of the low speeds and flat courses, but I certainly would never do that with a road bike. For what you'd pay for a disc conversion, you'd probably come close to what you could buy a new disc-compatible cross frame.