The below deck tanks are not UV stable which is why they are not rated for above deck use. If its not out all the time, maybe its ok, but if this is a permanent install, I recon its a pretty bad idea. Better idea to buy one of the above deck tanks from Moeller which will only need a fuel line since the vent is built into the cap.
http://www.amazon.com/Moeller-Boatin...deck+fuel+tank