Please be advised that the hood is the least of the problems when doing the 400 swap out. You must rework the front motor mount.
More thoughts on repowering an M809 series truck:
Add the cost of a different bell housing, fabrication of a new oil pan, , possible relocation of the oil pump pick up due to the new pan profile, air intake and turbo duct work and the relocation or the air cleaner etc, finding a place for the Jake brake pedal and dash control, quite likely needing a new radiator due to the engine making more heat under load than the M809 series radiator can handle, yadda, yadda, yadda.
Don't forget that the M809 series drive line parts are sized for the NHC250 and not for the exra power so it will be quite easy to twist off drive shafts or pop U joints. This will be especially likely if the truck is in loose soil or snow with the front wheels pulling. When the front axle gets to solid ground all the torque will go there and you stand a very good chance of breaking the front axle, drive shaft or transfer case.
As far as truck choices:
If you only the driven front axle you can buy an M818 and good luck to you in finding a sprag rebuild kit if the sprag is shot. Many if not most are.
If you need the driven front axle and the 45K rear winch buy an M916.
If you don't need the winch or driven front axle buy an M915 or M915A1 and think about mounting semi-offroad tires in place of the solid ribbed ones that will get you stuck on level ground with wet grass. The M915A1 has an air powered adjustable fifth wheel slider. The M920 adds considerable weight and a much larger turning radius to the mix (about 90' turning radius, from memory) and i think that the highway speed of an M920 is around 58 MPH.
From memory, the highway speed of a standard unmodified M915 is about 66 MPH and the highway speed of an M915A1 is around 56-58 MPH. Switching the M915 or M915A1 from stock 11.00R22.5 tire for the semi-offroad M916 tires (12.00R24.5) will help the highway speed some without being so high as to interfere with most tractors.
Expect a moderately loaded M915A1 to get 6 and maybe 7 MPG if you are not in really hilly terrain and you keep your foot out of the throttle and drive smoothly. I don't know what the M916 or M920 get.
You can buy brake and drive line parts for the M915 series trucks at any truck supply house. Finding M809 series parts in a hurry when your truck is sidelined and your income has stopped is tough to do.
If you buy an M915 or M915A1 you can go to a heavy truck bone yard and buy a good air ride rear bogy with better gearing for about $2K sometime down the road and get both better road speed and a lot better ride quality.
Lance
