You don't need a bigger carb to blow through than you would put on the motor naturally aspirated. Sometimes a smaller one actually works better. If you have the 390, I would definitely use it, as the price is right. I'm an edelbrock guy, and they can be used, but the lack of solid floats become a liability after a certain point.

"The first rule of overkill: You can never have too much overkill."
"Overkill is underrated."