Thomcat,
I'm really digging you're small diesel 5th wheel project but I wonder if a Dakota is the right truck for the job. I'm interested as to why you chose this dodge as you're base.

I've had similar thoughts in the past and settled on a Willys/Jeep FC170 dually as my ideal base for a small 5th wheel. They're not easy to find but they offer so many advantages with their 4wd cabover design over conventional truck layouts.
The OM606 is so much more powerful than the OM617 so I'd stick with that. 
Mating with a manual can be challenging  (where I'm currently stuck) or costly but can be done with bellhousing chop & weld.


Would you use a Sonoma again? I've inherited a low miles 03 with 2.2 auto & have been trying to sell or trade it locally for something more fun/diesel.

I'd love to hear more about your trailer too!