I was thinking a higher stall converter.
I ran a Dyno program and put in your specs. Granted this is not 100% acurate but you will see the power difference at different RPM's.

3500 308 HP 463 TQ
5000 495 hp 520 tq
6500 562 hp 454 tq

Now I haven't messed with any high rpm stall converters like this but, by the looks of these numbers I would say about a 4500 rpm Stall. Right now you are locking up close to you lowest hp range. Pluse those big cams were ment to produce power in the upper rpm's.