Plastic boats are very susceptible to warping fron pressure points, especially in warm weather. That's why you need to use saddles. Yakima makes towers and clips to mount a bar on the roof. Do the 'fit my car' thing on their website. A full set of 2 bars/4towers/4clips/4locks goes for ~$335. Then you'll need a rack for the bed. Thule makes the best for an open bed. I'm using the factory-installed tracks and Thule bars/towers on my Leer shell plus the double-bar Yakima setup on the cab. With the 4 bars (total) I use four pairs of saddles for each boat. My plastic surf boat has the same warping potential as yours, the 19ft fiberglass boats have enough memory to resist warping. My 18ft Kevlar boat is bombproof in the water, but over-tightening the straps will crack the gelcoat
So I use lots of saddles(located under the bulkheads). The Yakima saddles have better mounting hardware. Don't underestimate the lift that a large kayak generates. I've seen raingutters and factory luggage rails ripped off by the wind. Finally, the massive cabover racks are more flexible than your boat and will chafe the shiite out of it. For sure you need to get that thang away from where the boneheads can wreck it.