My opinion is that you're going to have to close off the back...extend the tailgate up to the top of the board basically. Cut a hole for a piece of PVC to go from the blower tube to the inside of the truck. You can hinge that part so it can swing away when dumping.
I could be wrong, but you're probably going to get too much blowout if you don't close the back off. Then get a black mesh tarp to the cover the top so the air can escape.