The biggest problem you would have with this swap is that the Duramax is heavily dependent on it's computer controlled systems. You cannot even get the instrument cluster to work without the ECM. Not saying it cannot be done, but very expensive mods would be needed, on top of the very expensive engine and tranny. The Duramax swap has been made into the next generation('73-'87 trucks)at great expense, but there is also a bit more room in those engine compartments.
I'm currently building up a 6.2L early diesel, will turbo it with a 6.5 setup, to put into my '68. The 6.2/6.5 is roughly the same size and weight of a BBC, and uses the same mounts, making it an easy bolt into the '67-'72 series trucks. These engines ARE NOT the early Oldsmobile designed gas conversions. They were designed with a clean sheet of paper at Detroit Diesel. That being said they are still considered "light duty" diesels. Don't expect them to pull like 5.9 Cummins. The best part is they are cheap and plentiful. Plenty of good forum support, like here, is available on the web to help with any problems. They can't be beat for fuel economy. The 6.2 in my '82 one ton dually gave a consistent 21 mpg. I expect 24-26 mpg in the '68 with a 700R4 and 3.07 rear gears 31X10.5 tires.
