Is this because you have forks that aren't drilled to accept a sidepull but have 990 lugs?
I'm not sure if you're meaning U-brake when you say canti-lever. If you mean the ones that you find on MTBs and BMX race bikes, those brakes have their lugs closer to the dropout. I'm sure I've seen plates like the Boomerang for fitting them onto non-equipped frames or forks.