convert to an automatic

I`ve got a 1985 chevy k10 truck (4X4). It origanally had a 4.3 litre
engine and a 3 speed with granny gear. I replaced the 6 cylinder with a 350 and now I would like to convert it to an automatic. My question is can it be done and what would I have to do. I`m guessing I would need to change the rearend gears but what about the differential for the 4X4. Any suggestions would be appreciated.Thankyou