The Vortec is a new design, not the old 350.
All of the newer engines are a vast improvement on the old, mainly because GM discovered the idea that all the parts should be the same size, not just fit the the holes with lots of slop!
If you look at a 350 crank next to an MB crank, you will know immediately why the MB lasts longer. It weighs twice as much, for starters.
The 300D was first sold in 1975 (my brother has one, SN <29,000). It was the four cylinder block with a fifth cylinder added -- literally. Original design dates from pre-Hitler, had overhead cam in the late 1930's. MB also made countershaft crown gear drive valve trains, clutched blowers (half way down the synchro clutch engaged the Roots blower), etc.
GM just shoves any old crap out the door, in general.
Torque is the key to acceleration -- what makes our old diesels driveable is that the torque curve matchs actual driving pretty well -- usually peaks at 2400 rpm or so. Lots of fun to get the old Volvo wound up and let her rip -- the slow start confuses gasmobile drivers, and I run away from them as the old girl shifts up and takes off.
And diesels with auto trans a lousy in snow -- way, way too much torque. My Volvo is even a pain on wet pavement -- very easy to spin the tires (both of them, without positrack!). Horrible on ice or snow -- the turbo comes up and the wheels spin like crazy. I expect the 300D to be worse -- more torque.
I suspect a manual tranny would be much better -- more control over what the engine is doing.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!