Every converter builder is different. Thus I suggest calling several. See if there's anyone local, saves a ton of shipping time/money in some cases.
Nitrous will throw a wrench into the vanes when choosing a converter, so make SURE to tell them you are going to spray. Yes it'll be tight on motor, will loosen up some on the sauce.
Even better, go to the track, find some Ford racers and ask them. One thing I learned long ago was that a Chevy engine guy can't build a converter for a Ford powered car. Well they can but finding a builder who knows Ford's usually nets better results
And finally, if you have someone put something together for you and it works exactly like you want, consider yourself fortunate. It's almost rare to get one done right the first time. "Right" is subjective...many of us shop on a budget, and that's perfectly fine. Keep in mind that with converters, "usually" the more you spend, the better quality product you'll get. A good converter is going to be close to $1000, maybe more nowadays. Been a month of Sunday's since I've bought any.