Has this been leaking since you purchased the convertor, or has it just recently happened?
I'm surprised to see that on a convertor of this type, a full flanged hub was not used. Most of the high horsepower/high end/ high $$$ convertors I've seen in the past use a fully flanged hub for added strength.
If it's been leaking there since you got it, I think someone at the factory didn't properly check the weld for leaks. Poor weld penetration is often the cause for a leak to appear there. Also, if you can do it, check the runout on the hub, if excessive, the hub might be in crooked.
If you can check the runout, email me and let me know what measurements you get. On a high $$$ unit, the runout should not exceed .005 to .010. Factory specs are often .025 or more.
Good luck.
transman