[cobrashoch]

Camshaft, camshaft, camshaft.
385's thrive on lift. More so than any engine I can think of other than a Hemi. That flat spot you describe sounds like you are either running out of cam on the big end or the engine is leaning out. I would first fatten up the jetting (and bleeds) of the carb you have now, before changing to another carb. You will have to play with the jetting, etc. anyway with any new carb.
If it still runs the same way same way the next thing to do is the cam. You will be quite surprised that to find, I'd bet, that a .62 or .63 lift bumpstick will give you more bottom end torque as well as a much better high end charge.
It also sounds to me like you've got a sluggish converter. It always amazes me that folks expect a 30 year old converter to perform like a new one. Those things do wear out you know. Sounds like the staiter(sp?) rings may be worn in your stock converter. And most tranny shops don't replace those rings when they split a converter open for a "so called" rebuild either.
A slightly smaller new performance converter will give you more torque multiplcation on the bottom end too, over the stock converter. We've gone somewhere else since the 70's on converter designs you know. I would also change the tranny fluid to a high performance fluid such as Valvolene high milage fluid or better. And yes, you need more gear too. Good luck.
cobrashock