400 BB build issues
Just reassessed another possibility--you said it's covered in rusty water now from the geyser-like expulsions from the radiator...
Did the block sit for a while and have garbage in the jackets before you got around to putting it together?
Are you positive the radiator is unclogged in any way? If you have a bunch of rusty water coming out, that usually means you had debris somewhere in the cooling system or it was sitting for a while without being cleaned before running.