hello, just to let you know, if you want to swap to a 4 speed, you will either need to get another NP205 that was originally behind a 4 speed, or get the 10 spline input shaft from a 4 speed NP205 and swap it into yours. i believe the NP205 behind an automagic had a 27 spline input shaft from the factory.
also, the SM465 and adaptor is most likely shorter than your TH350 and adaptor, and if so you will need to lengthen your rear driveshaft and shorten your front driveshaft, and you will need to move the adaptor crossmember forward, too (drilling new holes in frame = yay!
). sounds like fun, huh? I am going thru something similar right now.
I have a spare 4x4 SM465 that I would let you have, but as it turns out the rear output in mine is going bad so I will need to swap trannies in the future now too.
good luck finding what parts you need, and be careful of prices...apparently unbroken SM465/Np205 adaptors are made of solid gold or something ($$$).
EDIT: I forgot something...you could always try looking for a later model (i think its '86-'91) SM465/NP205 setup from a K/30. this setup is probably best because it has a 30 spline input shaft (vs 10 on the older setups). I also think this setup has a different adaptor that eliminates the coupler sleeve inside the adaptor itself. this is GOOD because the sleeve just creates more slop in your drivetrain, and causes unnecessary wear on parts (which is why I need to get a new tranny
but then again it did last 35 years without breaking). good luck on your swap.!