I would empty the radiator into a pot/pan, and start a fire underneath to bring the water to a boil. Then I would take a piece of plastic sheeting or thin metal that could be manipulated (aluminum foil, gutter, etc) and hang it over the boiling water at a slight sloping angle toward a bucket or jug that would collect the water. As the steam rises, it will condense on the plastic, and eventually drip in to your container.
I would feel comfortable drinking as-is if I were in a pinch, but I would really want to pour it through a 5 gallon bucket packed with layered charcoal and sand. At the bottom of the bucket I would cut a hole, and collect the water as it drained out. Better than Mama used to make.