I'm all for the allison tranny swap, but I personally hate diesels.
I can't stand the sound of them, or the excess exhaust they put out.
Those things pratically run off of liquid sludge. However, my dad has a 2004 Chevy 3500 with D-max/Allison combo and it hauls like hell. I've been trying to get him to purchase a switchable tune, but he thinks it will over-work the engine. I heard of people getting 150 hp Extreme Bully Dog tunes, but they said you cant leave it on very long or it will blow.