Unless I misread earlier posts, I think there is a bit of confusion. Though I'm speculating a bit, I may be able to clear up the Ultraman misunderstanding.
In the book, Wade has a choice of robots to choose from (one of which IS the Iron Giant) and he chooses Leopardon, the strange Spiderman/Giant Robot character from the obscure Japanese series. I suspect that because it IS so obscure, and the fact that it's tied to Marvel's Spiderman, the reference would be lost on the masses (and very confusing) so the choice of going with a well known giant robot was more favorable (not to mention Spielberg loves the character of the Iron Giant).
Second, at a completely seperate event in the book he chooses the Ultraman character to turn into for the fight (using the Beta capsule which is only good for three minutes). Ultraman IS a powerful giant but not a robot, more of an alien super being.
I think most people are just getting super giants fighting and assume they are all robots. So my assumption is BOTH the Iron Giant and Ultraman will be used.
Unless I'm wrong. :lol