I've truck topped a canoe for a number of years and the location of your bars might be problematic. The key is to have the front and back tied off to front / rear bumpers or hitches. If you leave the front of the canoe short of the front of the truck, that front rope gives the canoe uplift support as well as anchors the canoe from sliding back. Rigging an anchor rope on the back of the canoe in a way that prevents it from wanting to slide forward under heavy braking is also a good thing if you can figure that angle out.
