You realize that the TH400's tailshaft needs to be swapped out...right?
Again, not sure about the crossmember, but you will need to change your driveshafts, shifting linkage etc. Also the particular NP205 and adaptor housing you need is fairly rare.
I would save the time and hassle, simply fiind the correct NP205, and go dual transfer case. you keep the same factory adaptors, the driveshafts will probably fit, and you dont need to hunt down half of the rare stuff. Theres ORD, WMS, and a guy on the PBB (he does it cheapest) who can outfit somthing like this for you. Not to mention you wont have to rip the trany apart.