When I was doing my research, and finding out what I needed to do to get the most efficient trucks, I talked to many different sources. The suggestions I followed I were based on what made the most sense to me. My choices were Freightliners with CATS or Detroits or Volvo's with Cummins. I wanted 13 speed transmissions and the only thing I could get with that was Freightliners with CATS. The advice that I was given was that the CATS were best suited for the ECM upgrades that are available, and when you add the torque and horspower that we added, the CAT would be the best motor in the long run. I spoke with a man that did some of this tuning on an MBN and it is still running strong at 1.5 million miles with no onerhaul done yet. I believe that as long as I get good drivers, and give them proper maintenance I will see that kind of service also. I like my solid 7mpg, and my drivers like 600hp and 1850 ft/lbs of torque.
Do I have proof? NO! But I felt it was best for what my choices were.
Now to better answer this posts first question, I guess if I was looking to stretch a class 8 to make a D unit I would look for an 07' Volvo 780 with a Cummins and auto trans. You don't need what I have in a D unit, but I wouldn't be afraid of a CAT, especially if it is out of warranty...