Good stuff James!
On the 6.5 once you address the cooling - the castings are real sweet. Without address - the head likes to stress crack where the precombustion chamber, head surface and cylinder wall meet up - even when coolant never goes over 225. I also recommend a GOOD harmonic balancer on it - with high miles they tend to crack the crank at the #1 main rear radius due to accessory load stresses.
The Duramax had not been produced by my retirement so I can't form an opinion based on service issues to compare.
My opinion is ANY diesel should be 125% on the cooling system - no margin for neglect - they just have harsher heat cycles which makes for brutal expansion cycles of the parts. Maintaining a constant temp becomes critical - your system should "ride" the thermostat (in other words have the ability to cool below the thermostat's value with the thermostat fully controlling temp)!