Torque converter for 340

Here's my two bits. The Duster I bought new was a 340/727 with a 3:55 Sure Grip. It served as a grocery getter, holiday wheels, back and forth to work ride, the occasional jaunt down the legal or non legal 1/4 mile track and anything else I wanted/needed from it. It did all these tasks without a hitch.
Now in your case, you don't say what your overall use of the car will be. AJ has made many suggestions and he's right. Determine what you're going to be doing with the car and go from there. There are many good converter builders out there, and if you give them ALL the right information, they can spec. out what converter will work best for you. Don't let price be your only guide though. After almost 30 years in the trade, when it comes to converters, typically you get what you paid for.
Avoid used converters as you don't know what you're getting, it may be a gem, or more than likely a time bomb waiting to let you down at the wrong time.