Part of me is just being a cheap SOB, but I hate to "waste" HP and thereby fuel if I can get the same solution by reversing the existing fan.
As I sit here and ponder the alternatives, I was thinking that I could pull out the screen in front of the radiator and replace it with a pan (similar to the a cookie sheet without the bottom lip) and although the air would be restricted to only suck from the bottom, I would not be sucking air from the cab. To make up for the drop in cooling efficiency, as mentioned earlier I could install a couple of these with very low amp fans for under $80 for everything. I like these computer fans because they perfectly fit the 120mm fans which are low amp fans and they have easy mounting options.
Hardware Labs Black Ice Pro 120 Radiator (1/2" barbs)
Here is a whole page of choices
Water Radiators and Accessories
After everything else, this may be the best way to go.