Good question.
OMM also mentioned cam timing. If that hasn't been verified, it needs to be.
The first thing that jumped at me was how little lift he is running. That's only ~ .030 more than I run and IMHO he should be netting .700 IF his valve gear isn't jacked up.
I hate to be THAT guy (but I have to be because I am THAT guy) I think the OP needs to take a short break, pull the engine and put it on a dyno. Then take it apart and see what you have, then fix what you don't like and then back on the pump again.
Take those numbers from the dyno and all the other specs from the engine and car and send the converter back to whoever made it and make them get the stall up where it needs to be.
I have a personal animosity to converter guys. Too many times they end up with slower stall than the engine requires.
At this point in time, a 5500 converter is essentially a N/A street converter.
If you ain't data logging it makes it hard to nail down actual converter performance.
Converters matter.
Ramble off. For now.