After seeing some internet rumors about HD Tundras, some with Cummins V8 diesels, DRWs, etc, I'm wondering: Do you know anything official from Toyota? Do you have a link?
I watched a video on the Cummins V8 and two negatives stood out to me - it uses a CP4 injection pump and it has a timing chain instead of a gear setup.
Other than those issues it's a really appealing powerplant that offers plenty of power and probably pretty good fuel economy. Those issues would give me pause, though.