Can the torque converter be emptied or does it have to be rebuilt for your 350?

You can empty it out and change the fluid if there is nothing wrong with it and the stock stall speed will still work with the engine you will be putting in front of it. It's basically a fluid coupling. Since it's full of fluid and the fluid does most of the work, there is very, VERY little wear inside a converter. The only exception is if you have a lockup torque converter. The clutches inside lockup converters DO wear out and ultimately fail.