This sounds extremely fun.
Also, it might be great to use 1.4’s superflat customizations to generate an empty world and then copy-and-paste it to 1.3 so you only need to build, no need to erase.
What would you do about Nether?
If you disable it completely, there is no way to get to potions. However, if you set it up to be 400×400 too, it’s not certain that it could encompass a stronghold. I think it’s safer to dispose of it.
Would you leave the roof open? Allowing sunlight would prevent people from being owned by super-spawning rates of monsters. You could WorldGuard the whole thing so that nobody can escape.