Regarding the I-6 VS V-8 engines:
The I-6 has less inherent vibration than a V-8 because all metal part movements are either circular or in one plane. Ditto for an in-line 8 (old Buicks, for example) and for the "Boxer" engines of Porsche, Subaru, and BMW motorcyles (I owned a 1978 Subaru with 4 cyl boxer engine. Numerous times, I would hit the starter when the engine was already running becaue it was so smooth and quiet you could not tell it was running. ) . V-8 engines have metal engine parts that are circular and move in two planes. For that reason, many of the early Japanese cars used counter-balancing weights to get their V-6 engines (and some other models, I believe) to run without little or no vibration. Vibration or the lack thereof is just one of the things that can lessen or prolong engine life.
I-6 engines generally have a longer stroke than V-8 engines. This lets the engine develop torque at a lower RPM than a V-8. (look at the torque curves for a Ford PSD and the Cummins CTD and this will jump out at you. V-8 engines devleop torque and horsepower at higher RPM's. RPM's equate to wear over the long term. IMHO, lower RPM's at a given speed generally equate to long life.
One of the problems with getting to be as old as I am is that many things I learned to be facts when I was young have been changed by technology. My greatest experience with I-6 engines were Ford 300 C. I. D. 's and Dodge 225 C. I. D. slant sixes. My greatest experience with V-8's was with Ford 302's. The Ford 300 I-6 had more torque over a wider (and lower) engine speed range than the 302 V-8. But that 302 V-8 would wind up very fast to get to the rated horsepower (the 289 C. I. D. Ford would wind up even faster). That's why many of the early stockers used 289 Fords.
So, it appears that the V-8 has a place in cars and non-working trucks, IMHO. The I-6 (Lord, I wish I could still get an I-8) engines are lower RPM working engines and they have a place in working trucks.
One of the things I saw 20 years ago at a dealership in West Palm Beach was a salesman put a glass of water on the hood of an I-6 and a V-8 Ford 1/2 ton work truck and the water in the V-8 shook a lot more than the I-6. That convinced me that in-line engines were smoother. Would be interesting to compare a PSD and a CTD today... .
All that being said, I've been driving Ford PSD V-8's and 302 V-8's for the last 24 years for my PERSONAL vehicles. That's been due to the fact that the I-6's, when they were still in production, were always in bottom-of-the-line work trucks and I wanted a nicer vehicle for my personal use. Obviously, I've put a ton of miles on V-8's with no problems over the years... ... due to obsessive maintenance and tender care when driving them. So they are good pieces of machinery.
I, though, am convinced that Navistar and Ford have made a mistake with the 6. 0. I may be wrong, but it's my money and my money will not buy the current V-8 diesel engines on the market.
As for the Chevy, if memory serves me correctly, they had a lot of problems with head gaskets between 40 & 70 thousand miles when they first came out... due to aluminum heads, I believe. As I recall, their fix was a different gasket, which was thicker. I was a field sales engineer for the 2nd largest aluminum company in the world before I retired. I was a proponent of the use of aluminum ALLOYS for many uses that were justifiable in the marketplace. But even I don't have faith in aluminum heads, gas or diesel. Heck, my wife's '01 F-250 CC has aluminum heads and I still don't think they'll last like cast iron... but that's what she wanted, so..... (She doesn't pull anything with it, just grocery shopping and hauling people!)
Physics comes into play with aluminum heads. Aluminum, in all of its alloys, expands and contracts at a different rate than iron. There's no way the block and head of a Duramax can get around the basic physics. And no way I'll ever put my money in one.
So you pay's your money and takes your choice. For my money and my use, I'm voting with my checkbook on Cummins and Dodge for the next 6 or so years.
Florida Ed