In factory form, the 4L80/85E's only came in van's along with a pretty severely detuned engine.
Nothing is factory in this case, but the 4L85E will fit under the factory floor pans. Any Chevy patterned trans will fit behind the Duramax or LS-x series engine. One of the brilliant things GM did with these engines. Conversely, the Allison would bolt up to any Gm engine. I don't know what you'd have to do as far as spacing for the converter/flexplate issues though.
I'll have some pics as the work goes on, but I'm trying to keep this on the down low for right now since it will be a number of years before the car is finished and there's nothing worse than someone with no imagination but more money than you stealing your idea before you finish and getting the credit. Not to say that there isn't already a couple of cars out there with a Dmax in it, there's a 1966-67 Nova wagon in the latest issue of Diesel Power with this setup already.
Remember, I only spend so much per year on the car, so it's not going to be a straight through type of build someone can follow. I'll have a new baby at the ned of next month, so I'll be slowing down on my projects for sure.