I ran one and did nothing about the gap. Never had an issue, aside from the bumper hitting everything I wanted to drive over.
I had a quick look at your profile and in that picture you can actually see the lower ramp that feeds air to the bottom of the radiator and a touch of the radiator body as well.
BTW, did you find that your truck ran slightly hotter with the ARB? I ask that because without covering that hole the compressed air can escape down instead of the more resistance path of passing thru the radiator body.
thanks in advance for a reply and if you can post that picture on your profile i think it might help others visualize the potential issue.
good to hear however you did not have an issue