Well we now know the RPO for the new baby Duramax is "LWN".
Initially, it will be released on crew cab models only. GM hasnt said if extended/regular cab models will be added down the road.
Now before you all start whining "oh boohoo stupid GM why only on the crew cabs!!!", probably the reason GM did this is because it costs millions of dollars to validate/test a powertrain package. And then they have to re-do that testing/validation for every chassis configuration (ie, vehicle length/cab config).
You have to do handling tests, frame testing, towing, drivetrain testing, stability control calibrations, ECM calibrations, TCM calibrations, crash testing, the list is endless... And then you have to do that for each and every cab config. Shits not cheap!
So thats R&D testing cost, man hours, etc (times 3) to add it to all cab configs. Thats just not feasible right off the bat, for an untested market (midsize pickup truck diesel). GM doesnt know exactly how many they will sell, and what the market share of the diesel version will be...If they spend that millions of dollars (times 3) for all 3 cab configs and only sell a few thousand a year, they just wasted a lot of money.
I would say once its released, and if its a huge success, then they will focus on integrating it into the regular cab and/or extended cab.
Why crew cab? Because like 2/3 of pickup truck sales are crew cabs; market/sales share dictates which cab configs get the most attention as far as pushing new features to market...thus crew cabs historically/traditionally get the expensive new features first (like side curtain airbags, stabilitrak, etc). Thats the way it is. Blame consumers, because most of them buy crew cabs.
And now I know what you're going to say next "but but but why not the regular cab because it would get amazing mileage!!!". Because probably 98% of regular cabs are purchased by cheap-ass fleet operators who could care less about the diesel and just want a bare bones inexpensive work truck. They (penny pinching fleet customers) dont care if the diesel would potentially get 5mpg more, BECAUSE it burns more expensive fuel, its more expensive to maintain, and costs thousands more in initial purchase price......
Ben