This^^^^ the best choice for a converter NA is not the same converter that will work best with N20.
Sounds like you're planning to make a bunch of changes at once. I struggle when I've done that in the past. IMO a nitrous car needs to have gears picked to get you to the line at the rpm that you want - hopefully somewhere just past peak HP rpm because it's 'all about the hit' off the line. I'd pick a converter to be 200-400 rpm's below peak torque on the hit. These imply that you know where you motor's TQ and HP numbers are peaking. It's a great idea to find a converter company/guy that you trust.
Something to keep in mind, you can change gears in a weekend, amount of nitrous with a nozzle or setting change in less than an hour, but to change stall you have to send the converter in and wait.........waiting kills me lol.