I have a Figure 8 NP205, so that's what I needed to do. I swapped in a long 2wd output shaft when I rebuilt the 4L80 and added the rear reluctor gear at the same time. The TH400 I am replacing has the long ~4.5" output shaft, so you have top do that. On an 80's NP208, you don't need to do that. He will have to swap the input shaft on the NP208 to a 32 spline input and then the stock 4wd 4L80 will bolt right up using the stock 4L80 adapter. No adapters or anything needed.
Personally, I would just get a junkyard 4wd 4L80 with the adapter, wiring harness, and PCM for a couple hundred bucks and swap it all in together. You just need RPM and TPS inputs from the engine and you are good to go. Buy an HP tuners, EFI live, or ELM327 adapter and tune it to your hearts content.