Since it will be mostly street, a 6.4 variable runner manifold is my choice, which I bought new/takeoff. I have a 5.7 core, plan on 4.00 bore with Apache heads and a 3.888 stroke for about 390 cubes. A 4 inch bore will leave you some safety room, and a variable runner intake ought to pull the best torque up to 5,000 at least, then I would hope it will match some great intakes above that. The 5.7 newer variable intake might be better for your application. I went with a 6.4 since I got a good deal on it and the angled inlet is going to fit my chassis very well. Since a stock 6.4 makes 485 hp, I don't see why it won't be a great intake for up to almost 600 hp?
One last thought; you can set the runner length change up on an rpm switch, and tune it to what any size motor would want. Factory is 4900 rpm for a 6.4