Hi RexW. We have a '98 Expedition and have always used the factory rack on it. We carry a big and heavy Old Towne sport square stern. We have never had any trouble, nor even a hint of concern.
We separate the crossbars as far as possible. We use two web straps over the canoe fixed to the crossbars. The rest of the rig includes two stabilizer ropes at the front of the canoe that go to web strap loops we've attached to fender bolts (just under the sides of the hood), a rope from the front of the canoe to a tiedown at the front bumper, and a healthy nylon rope from the front of the canoe, along the rooftop to a tie down point on the rear bumper.
The order of snugging is
1. Stabilizer ropes, to center the canoe
2. Front tiedown
3. Rear tiedown
4. Web straps
It is VERY secure. I laugh in the face of semi's. The Expedition sways more than usual when we meet one, but the canoe does not budge.
Jim