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A very doable swap, the diesel versions were the L8000's. However there s a bit more than just a engine swap, you will need to deal with the gearing as well. If you have juice brakes, you will need to deal with a vacuume pump as well.
Not sure it really is worth the effort though, unless he L800 you have is incredibly good condition, or set up as a one off type rig that has a specialized function as already set up L8000's with the 3208 are readily available. Many older good diesel trucks are coming out of CA as they cannot be registered / used without modification to meet the particulate standards, and that is if the manufacture even offers a retro fit kit.
I think if I were going to put forth the effort and cost to do a diesel swap it would be a with something that has a bit better reputation, and more importantly is a linered engine so rebuilding is practical.