I have a 14' Sportspal canoe in the bed of my extended cab Dakota right now. I can drive it up from the boat ramp without even strapping it down. When going down the road I use one ratchet strap at the back of the bed and one at the front. It doesn't even move. I drive down dirt roads and do 80 on the interstate with no problem.
For insurance I ran a piece of aircraft cable between the tie downs next to the cab and secured it to the anchor loop on the front of the canoe with a lock. That way, when the straps get old or rot and break loose, the canoe might bounce around a bit, but it can't leave the bed of the truck.