You'll get more out of a disc on the front than you will on the rear.

Adding a disc brake to the frame won't be a prob,but you will need to either replace the rear wheel or rebuild it with a disc hub. You'll also need to replace that rack. If you're running canti's,you'll need to make sure you get a 'road' disc caliper to match the current short pull levers.

If the problem is the current brake getting in the way of fenders,then a much easier and cheaper solution is to just swap the brake. Canti's come in different styles with arms that go from almost vertical to completely horizontal. Any shop or even co-op should be able to set you up with what you need. Also,how is the brake interfering with the fender? Are the arms or the wire hitting? It's possible that the straddle wire just isn't set up right.

Post a pic of the brake with a fender.