It sounds like a stellar build, and you already have your mind made up. I believe an 800HP 1k HP 5.9 with stock reliability only exists online. The 466 is a monster of a motor compared to a 5.9, bigger and stronger in every way, and pullers run the at crazy HPs, too, but none of them would do it in a plow tractor running loaded all day. Boats are an excellent place to look for maximum HP as they have unlimited cooling. You get to see the maximum real-life reliability HP for most engines. I believe the P pump 5.9s topped out at around 350HP in boats. Another thing to note is that even a 5.9 at 400HP/1000ftlb, the maximum HP dodge in running in a 6.7, it will make much less torque than the Cummins 250 until around 1600 rpm or more.
I do have my mind made up.
A bit of a caveat: I never made the claim that a 1k hp 5.9 cummins would have the same reliability as stock. I said that you can push them to 800-1k hp before they start loosing reliability. I would say getting 250-300k+ out of a highly modified engine is quite impressive, and I would call that a pretty reliable engine. Pushing these engines to 500hp has a long list of people getting 5-600k++ miles off of the engine with minimal work like replacing things like injectors. I would say that is an extremely reliable engine. An unmodified 5.9 that's taken care of that can get over a million miles: I would say that's an insanely reliable engine.
As far as boats and 'unlimited cooling', it seems like you're really talking about Full Duty Cycle. Very few engines will run full bore at stock and get 100% duty cycle. I'm unfamiliar with the boating application of 5.9's, but trying to compare this specific application to automotive really isn't an apples to apples comparison. I really don't know the point of going into all that, nor do I know the actual specs on it.
I'm going to use this truck to cruise around in. Maybe tow a lighter trailer, under 12k. The top speed of the truck itself is about 55mph. It's being modified into an RV, so it will not be hauling things. It's not like it's going to be seeing full duty cycle. It's going into an application that mostly came with a Multifuel engine that is VERY gutless. Turning up the fuel screws gives a 5.9 more power than the multifuel had. A 5.9 should be able to push that truck at an estimated 300HP full duty cycle easily. Modifying it more, and being able to get there faster and maintain the same thing easier: I'm really not anticipating this engine is gonig to be working all that hard. If it does I can always go from there. It will have the same radiator as the Mack, so it will have a lot more cooling than a standard 5.9, and they tend to do pretty well pushing much, much more than what I'm trying to do with it.
Also, I alread stated: I put a towing cam in it from Hamilton that will effecitvely lower the torque band in the engine and put it more in line with the Mack and the Multifuel torque bands. It helps the turbo to spool at lower RPM's, helps to lower EGT's, improves economy and adds a bit of extra horsepower when coupled with other modifications such as injectors and larger turbo. I'm going to go with a larger turbo, as that is where the lower EGT's will really kick in, and it will get a bit more out of that lowered torque band.
had a rating of 195@2800 and 425ftlb@2000. 
