If I were to do it again, I would use the water pump relocation plate and move the fan up to the center of the Radiator. It has to work better in the right place.
That is most likely your problem. There's a couple of things you can do to help the situation besides the relocation plate (though that would be the best solution). One would be to add a shroud around the fan to make it pull the air through the radiator rather than from the sides/bottom. The other would be to add an auxiliary electric pusher fan to assist the main fan, particularly when the truck is sitting still.
