I need to build something that will sound like the riff in
Metallica's "For Whom the bell tolls".

Any suggestions?

By "the riff" do you mean the intro upper register bass riff over the rhythm guitar, or the repeating guitar riff played by Kirk that comes soon after?  Or do you simply mean the rhythm guitar sound?

You're referring to the crunchy rythm sound that accompanies the word F.W.T.B.Tolls.
 Questin...what kind of amp are you trying to get to do this?