So the width of your canoe is ok but its really long?
Just make sure you use ratchet straps, 2 up front, 1-2 over the middle and 1-2 for the rear. It is a little scary the first time you hit 50mph with one up top, but if you are strapped down right, you will be ok. I got some of those pads from Sportsman's also, they help a lot.
Bart