I used Dave Husek for my Grand National converter.
http://www.turbobuickperformance.com/
Not sure if he can hook you up with a 400 trans or not but his converters do what he say's they will.
Not sure what kind of shift points you will be using but that motor is going to push through an average over the counter performance converter.
I had Dave set my up to couple at certain rpm and it does the job. Definitely get someone who will set it up to your combo or you're just wasting your money. Not cheap for a good converter but defiantly well worth it.