I wonder what you like or don't like in terms of how it drives now? Is it healthy? Do you travel/cruise in it? Mostly around town? It sounds like a pretty well sorted engine combo to me. A set of open chamber Edelbrock heads (88 cc's from memory) would help it breathe and should be (basically) drop in replacement for the 906's and work with the domed piston (but check!).

Personally, I'm not a fan of buzzy gears or slippery converters unless racing is the main objective. 3.23's work well and a 440 should have the torque to handle them even with some cam. OK at highway speeds and around town. Even the original 318 in my Challenger handled 3.23's well.

Perhaps there are other areas that would benefit from attention. Suspension? Tires/wheels? Exhaust?