Get it dynoed with the factory converter locked via programming. Determine true peak torque rpm. Buy a converter that stalls 500 rpm less than that number. Just be sure the converter manufacturer knows your actual numbers and all the vital static's he needs.
Once it's installed the only way I know to be sure you have the correct stall rpm is with a transbrake. If you don't have a transbrake then log a launch. Determine what rpm is reached immediately after launch. You should reach nearly peak torque within the first few frames. With my stock converter I'm 2.33 seconds out before peak torque is reached. Not very good for drag racing. But I can launch a 1.95 on stock radials with this soft launch. With a correctly sized converter I would be all over the track with radials.