I have heard a couple people do it, they use eletric fans from ford Taurus's. They dont haul anything, and just drive around empty though. Since the 6.9/7.3 is an IDI, part of the combustion is going on in the head, this means that a lot of heat is transfered to the coolant. IMHO you need a really good fan, mechanical IMO is the best, to keep things cool.