Its been done before, I think I seen someone on Cardomain that did a complete engine trans interior swap from a 05-06 SS into a base 00-05 impala. I bet its very time consuming though, but its surely doable.
I have also seen somebody's car who has done it. I don't remember the details, but they said it was not nearly as difficult as it looked. I am fairly sure that Don and I are talking about the same car.
Here are youtube videos of a 2005 Impala base with the 3400 v6 changed to the 5.3L v8 used on the 2006-2009 SS. He also changed the interior dash and shifter to a 2006 Impala SS. The guy is no longer answering questions. I posted the youtube videos an another question, just click the link,