I think you may have frozen your whole cooling system as you indicated had crossed your mind already - Get it inside a heated space quick before you have permanent damage. Yes it can and will freeze that quick.
But first: are there plastic shipping plugs in the radiator hose barbs on the new unit?? Check quick and get the water out of there!!! I hope this comes out ok for you, really. But you are too far away fore to help other than this post.
How many old threads actually convince *you* to read all 36 pages? http://www.jeepforum.com/forum/f12/length-envy-build-thread-629700/