Beefed up My six, need more power to the Rear

I don't know anything about the Webers, but since you already have them & the intake you probably want to keep that. I would be leaning towards the 390 CFM Holley unless you're twisting ungodly RPMs. At six grand, a 230 inch engine is going to pull 399 CFM if it's capable of 100% efficiency. At 83% efficiency, it's capable of 330 CFM. I tend to like a strong Venturi signal.
I am with everybody else on the gearing. And other than the fact you already have it, I would probably be looking for a 904 or 4 Spd, less parasitic drag and less weight to haul around.