Just make sure you give all the info on your setup to whoever is building your converter. the links TGJ gave you are good places to get one. I am sure you will be happy with the results. Also remember to increase the cooling capacity with a aftermarket trans cooler if you have not done that yet.
The first time I drove my friends Z28 with a good converter and a well built tranny I could not wipe the smile off my face, his car pulled so much harder and it was not heavily modded it felt like a different car. I know this a camaro and not a mustang but same Idea.
Thanks for all the info guys! I will check in to some of the links recently posted. As to my screen name and avatar, I joined this board several years ago when I got my first mustang. It was a 94 GT also with an auto trans. My avatar is a picture of that original car. Sorry for any confusion. The car I am working on now is an 04' 40th anniversay convertible that was actually my wifes first mustang (bought it for her when we got married), but with a little one on the way now we decided to trade vehicles (sounded good to me after I started buying parts )