Might want to google Mojave heater (I think).....seen it used in several off road trucks. Might want to just block off the vents where they go thru the cowl, otherwise it will be like the vents will be open. The fab guy that did my cage was going to just fab up some sheet metal plates to seal the holes. Just found it
http://www.flex-a-lite.com/auto/html/mojave-heater.html