Looked into this swap a while back so I dredged up a couple of useful links for ya':
http://www.advanceadapters.com/tech-...rocess-208241/
https://www.novak-adapt.com/knowledg...tomatic/4l80e/
The second link is just some info on the 4L80-----Novak concentrates on Jeep products so the only NP241 info they offer is for an oddball one only found in Jeeps.
Advance Adapters makes an adapter to mate the 4L80 to the NP205, but it is a bit expensive------$670, and you have to start out with a 2wd 4L80.
http://www.advanceadapters.com/produ...h350-or-sm465/
The Advance webpage states that the NP208/241 was originally mated to the SM465/TH400/4L80E, so I'm inclined to think that your adapter will work. I also think, given your intended usage, that the 241 will be okay, but you might consider a quality rebuild. I read somewhere that it was used in 3/4 and 1 ton applications including military, and I know for a fact that a friend's Ram 3500 turbodiesel 4wd dually came equipped with one. There's a website out there somewhere that explains the numeric code for the New Process cases------the first digit is the # of speeds, the second is case strength (1 - 7), and the third is method of engagement (manual, electric, AWD, etc).
Also, are you aware of the New Process 260 series "pump rub" issue? If not, and you plan to use yours for anything, you should look into it (in case you decide to do the Dana axle route).
Will your big block be computer managed? If not, you'll have to get a separate tranny controller (also expensive) as there are no non-"E" 4L80's.