Simp, For me the novice, the BC 400 to fit in the old 250 slot requires:
1. Front and the rear engine mounts need to be modified to accommodate the larger engine size, 1/2" plate steel and some happy hockey pucks
...
Only the front mount
2. and aligned to have the engine sit to the passenger side slightly several inches??? to clear the front drive line.
Not necessarily
3. BC 400 needs a new oil pan to clear the front axle when hit hard or flexed heavily.
Big cam 1s use the same oil pan as a small cam
4. BC 400 needs remote oil filter mount to clear the passenger side frame.
No
5. BC 400 needs new bell housing SAE#1? If you wanted to keep the small but tuff 5 speed that is SAE#2 you could buy an adapter.? Has anyone done this before..i.e kept the 5 speed and liked it? Can it adapt to the SAE#1? if using an Eaton Fuller trans the T-case will need to be pushed back a bit? Could the trans and T-case from a M123 be used to save space since they are not a divorced set?
You dont have to buy an adapter for SAE 2 bellhousings
6. Hood modification will be needed as the engine is much taller (Or cab raised a bit like 4 or 5 inches?)
only if you're are planning on using #1 bellhousing
If M809 has a PTO shaft driven winch, the PTO must be converted to a hydraulic pump interface... I don't know much at all about this but I can only imagine this is the case as the shaft normally runs the right side of the oil pan and will not have line of site from the trans after the install... True?
If you use the stock transmission nothing moves
8. Exhaust will need to be modified.
yes
9. Cab heater core box will need to be moved to the driver side and in cab modded to match? ... (and swapped with the air intake housing that is also relocated...)
maybe
10. Intake air will need to be modified. Most likely to the passenger side fender.
its your choice
11. Fuel line will need to be modified.
No
12. Generator/alternator bracket will need to be made or modified to align and allow adjustment.
no
13. Intermediate trans to T-case shaft will need to be cut shorter.
not with stock transmission
14. Cab floor modified for new position of transmission stick shift shaft (not sure how much with the Eaton, or 5 speed)
15. Radiator hoses will need to be modified
yes
16. Radiator lower mount and upper mount will need to be mods
yes
17. Engine sensors will need to be modified. Oil pressure, Engine Coolant Temp, RPM sensor.
no, they all interchange
18. Starter cables adapted: Main power and start lengthened?
no.
19. New clutch, pilot bushing and bearing. Shaft for alignment.
yes if you went bigger
20. Flywheel resurfaces.
21. Clutch linkage modified
22. Fan shroud will need to be modified
And a lot of band-aids, hand cleaner, oil, antifreeze, not to mention blisters, broken tools, drill bits, welding wire, and a few tanks of gas running for bolts and other assorted parts.. LOL I will be here watching . Good luck. Take your time, we all see your busy.
didn’t think you could bend one like that 
