Been there, done that, except the boat was in the back when I was making a three point turn in the dark on a dirt road at 1:00 am. And it was a C-1 not a kayak. My conversation was like this -

me: Think we got room back there?
friend: No problem, back it on up.

SMASH, tinkle, tinkle

Grrr. At least it was only the shells' window and not also the truck's...

I clamp mine but have had better luck with 6 clamps than with 4. With 4 clamped to the bed rails (and the gasket tape) I got some leakage and it slid a bit. Now I have it with 6 clamps over the "rail guards" and it is solid and watertight (http://bookwoman.net/scott/truck/RailShellInterior.jpg).

I'm not wheeling as such, but I've carried some hefty loads on top on gravel/secondary roads with no problems. And that is with a no-name fiberglass shell.