Just a question - what is your motivation for juicing the HP, and what is your end-game?
I'd actually stick with the 8.3 unless you have a terrific reason to not do so. The 8.3 is rebuildable using an in-frame kit (new liners gaskets, pistons, rods, etc...) for about $1,100, and parts are everywhere. You can as at least one poster has mentioned uprate your engine by turning up the pump. If you are not planning serious military environment exposure (deep desert at 110+*F at altitude), without that abusive Private Ledinmyfut, or Quartermaster Cantadd, you won't need the extra margin they have spec'd in to the engine. Recall that the military COTS mentality is to take a commercial-off-the-shelf parts and make them work in severe circumstances, and that is done by de-rating the heck out of everything and over-building everything - it's also why the trucks weight more than a similar capacity recreational truck does (think about the gigantic spring packs as an example).
The P-Pump used on the 8.3 is the same style as the better 5.9 P-Pump (I believe it's the P3000 rather than the P7100), and quite a few of those parts are interchangeable (I have swapped a boost compensator from a P3000 to a P7100), making power adds rather easy. If you follow the normal power-add recipe of "Fuel, Air, and Lower-resistance", it works the same in smaller engines as it does in bigger ones - engines are, in their most fundamental concept, just air pumps. Bigger intake filter for less resistance, larger or compound turbos for more boost (ball-bearing if you want less turbo lag and shaft drag), intercooler or (bigger one) to cool the air charge, more fuel to match the higher air charge (same fuel/air ratio for complete burn), and a larger pipe out the exhaust for less back-pressure - and there you have it.
They do the 8.3 in the 500-600HP range in marine applications, the big thing there is the cooling (boats have an ocean as a cooling system). Just remember that any diesel engine is between 35-45% efficient, the rest of that is waste heat (cooling water ~30%, exhaust ~20%, ambient radiation is the balance). Diesel has about 128K-BTU/hr/gal energy content, assuming the absolute best efficiency (45%) you are using 1,144,995.11BTU/hr (~9GPH) for the power output at 450HP, but an additional 1,399,438.46BTU/hr (~10.9GPH) is just waste heat. Pay very close attention to the size and condition of your radiator if you plan on running close to your rated power at high duty cycle no matter what engine you settle on (de-rating an engine eases the cooling needs too). This doesn't take into account the parasitic loads like the radiator fan (15-25HP), or the drive train losses.
One last thing, take a good look at your transfer case before you take the plunge on the Allison - my recommendations are based on the engine to transmission interface. Assuming the transfer case is "divorced", it should be a drop-in, but if it is "married" be prepared for some work to adapt the new transmission to the old transfer case (I would hope in this case that you have long-term access to a machine shop).
