Tranny confusion......Manual to Automatic

Ok guys.....I currently have a 360 with a 4 Speed behind it in my 75 Duster. Car was originally a Slant 6 auto, but the previous owner changed it over to the current setup. Anyway, I would like to convert it over to an automatic, but I'm confused about what all I need for the engine besides a flex plate and torque converter. Does the torque converter need to be balanced? Do I need to worry about throwing off the balance of the engine (was balanced when it was built). Was thinking all I need to do is get the transmission, Torque Converter and a flex plate and bolt it up. But wanted to make sure I didn't make a vibrator out of my car.