The duramax has a ton more power then the 6.5, the people who want the D-max in the H1 want it primarily for on road driving. From what I have heard the H1 needs a two inch body lift to accomodate the Duramax.
The Allison is a VERY heavy duty trans with the proper computer upgrades it can handle 800 ft.lbs without having to rebuild it. The fifth gear is also a nice feature.
Its a dry heat if your breaking lots of halfshafts you should look into doing the 12K axle upgrades which can be seen on Scot Smiths website (
www.southernaxle.com) you will probably never break another halfshaft again.