Maybe go with an aluminum rad but with junkyard fans to save money. Maybe try to get fans from a big motor that needed a lot of cooling from the factory, like from a big block V8, or some kind of commercial vehicle (tow truck, ambulance, shuttle bus...etc.) Usually when they mount radiators in the back, they mount 'em at like a 45 degree angle. Higher toward the front and sloping down toward the rear of the truck.