The 208 is a pile of junk. They're far more expensive than a 205 seeing as they all break and everyone who needs one pays for it.
It's a bad design that's prone to case and shift fork failure.
I beg to differ on that one. I've had a NP208 in my 84 fullsize K30 for quite a few years now. It does see some moderate of road use like getting fire wood and the occasional juant through whatever to pull my friends out. My buddy has one in a fullsize blazer that gets the living crap beat out of it everytime it goes out.
With the advent of the 203/205 doubler the price of a 32 spline 205 has shot through the roof. That is what is needed for the TH400. The 27 spline 205 from the TH350 can be machined to accept the 32 spline input gear but the cost ends up being the same as a 32 spline 205.
The 208 has a small problem with the plastic wear pads wearing off the shift forks causing it to wear the aluminum shift forks and jump out of gear. That is easily enough remedied with a rebuild. If the aluminum case was such a big deal then the aluminum cases on the auto trannys would have the same problem as they support the T-case.
I've seen broken cases on NP208's, but those were from the full weight of a vehicle slamming them between a rock and a hard place. Hit any case hard enough and that will happen, even to a NP205. Others were from a lack of fluid in them.
If he really wants to use the NP205, then by all means do it. But the last adapter I seen for this combo went for around $300.00.
I guess its all in what you want to do. If it was me I'd stay with the 208 for less money unless I already had all the parts (which I do. Sorry not for sale.)
If it to pound on then by all means spend the money to make sure you don't get stuck somewhere. If not, then do it economically.
The 208 will more than stand up to a stock 4BT. A built one might be a different story.