I would go with a TH-400 3-speed auto if you can find one. I've got one bolted up to my NP-208 and it works great. I've also got 4.10's in the Diff's. Keep an eye out for ready to bolt in axle assembly's from your favorite wrecking yard. Swapping them your stuff will cut down on your cost.
Clayton
He'd likely have better luck finding a Th400 with an NP205 transfer case, much more common than the Th400/NP208, and would be in the race track pattern NP205 since the Circle Pattern NP205 and TH400 is even more rare than the Th400/NP208. Not saying they're not out there, but I see many many more Th400/NP205 combo's for sale than I do Th400/NP208 combos.
I myself, prefer a Th350 over a Th400, and even in a 4x4 model. The only advantage IMO to a Th400 is nothing more than strength. Past that, The Th350 has all the other advantages, better first gear, lighter weight, much more common and so easy to find including adapters and Tcases, Less hp loss at the output shaft, and shift points and shift quality is much more tuneable using the Detent Cable. Th350 has the same longevity of the Th400, but there's 2 things in the Th350 that although not required, but if you intend to be abusive to the transmission, I'd beef up the intermediate roller clutch and race to a sprag and hardened race, and a hardened input shaft. With those 2 mods, I'd say you're going to be very close to the OE strength of a Th400 as far as failures are concerned. The Th400 will still be stronger, but the Th350 is strong enough, unless you're over 500hp/500ft lbs, will hold up just fine with these 2 mods.