A TH400 adapter bolts up to the 4L80. The index ring on the adapter just needs to be machined down to fit into the transmission. The 205 needs the short female 32 spline input gear. Your male input NP205 will need the input bearing bore machined to accept the larger female shaft input bearing/gear. Output shaft on transmission needs to be change to a long full splined shaft then cut to correct length. Was typically used in RV's. You can source all the parts or get this from Advance Adapters.
https://www.advanceadapters.com/prod...h350-or-sm465/
Includes everything except the transmission output shaft. Adapter is already machined, your NP205 will need the machine work. I have one of the 4L80 output shafts that I am not going to use as I went the 6L90 route. May fit your transmission.
Richard