Also, after you installed the trex you could just get under the truck and screw additional supports across between the crossmembers if sagging IS a problem. By doing this you would not see the fasteners or the stiffeners from the top. Jeff is correct when he says the trex sags more than wood. But with all do respect I personally think it should be fine. I think I'm going to install it in my truck as well. I'll keep you posted of the results if I get to it first. Please let us know how it turns out if you go that route.
