Here is the official answer from a former Diesel manufacturer employee (Read that as a former Cummins Employee). The 4B and 6B family of Cummins engines are heavy duty, continuous use Mid-Range powered engines, while the 6.2 and 6.5 are automotive based light duty diesels. The Navistar (Ford Power Stroke) is also a light duty diesel, so is the up-comming LDD from Cummins (Light Duty Diesel) that will be powering the 1/2T Dodges
That being said, you can beat the pi$$ out of a heavy duty continuous use diesel 100% of the time, while a light duty diesel has a lower duty cycle, more like 10% at full load, 25% at 50% load, 25% at 25% load and 40% of its life at light to no load.
The 6.2 and 6.5 are not related to the Olds 5.7 variant that GM had in the mid 70's, so the block failure issue is no longer a concern
If looking at the 4B, check with your local Cummins distributor as you can get them for much less than any of the auction sites. Also the Echo engine (4-valve ISB variant) that was slated for Euro use was used in the US in UPS and Fed EX trucks in the early 2000's Much more power. Those engines have rear gear train and were the predecessors to todays QSB industrial engines.
Personally the 4B needs a balance shaft like the Deere 4.5 liter to prevent the 3rd, 4th, and 5th order harmonics from rattling the engine apart. The 4B is also known as the "paint shaker" as it shakes like a dog passing a peach pit. Although the marine double dampers help it. The 6B and QSB engines are bullet proof, smooth, and in non-100% duty cycle mode can make a crap-load of power with twin turbos, etc. Hence the superiority of the Dodge trucks with the Cummins Diesel. Unfortunately 6 cylinders in a row are a bit long for some conversions as compared to a V engine.
Unfortunately they all are assembled by humans and subject to failure eventually.
As Grigg stated, pick an engine that you will be happy with and live with that decision. In an automotive installation, both will run for a really long time with proper maintenance, and both will die fast and painfully without proper maintenance.
Scott
