t400 to t350

[WyattEarp18]

So I've got a 1979 K20 camper deluxe with a small block 350 and t400 and from what im told is a NP203 transfer case. I have a t350 and transfer case from my 78 K10 Silverado. I would like to swap the trans and transfer case for the t350 combo. I need the t400 combo rebuilt. Is there anything i have to do besides drop the t400 and swap in the t350? The NP203 I have now crapped out on me and is vibrating violently after 35mph and the t400 isnt picking up oil and kicks out of gear regularly. Any help and advice is greatly appreciated.