[Gordon Levy]

Your cam is to small to make more power. With the air flow you are capable of, you can use a far more aggressive cam profile. Your jets are also to big. Go with less jet but more pump shot.
Next would be the air cleaner. Most I have found out there will cost a substantial amount of HP, especially the oval cobra air cleaner. Exhaust can also be holding you back. primary tube size and the types of mufflers used can really choke an engine if not properly matched to the air flow of the engine.
I build many engines that are simular to yours and regularly get 530-550hp out of them. The 408 really likes the heads and intake you have chosen because of the volumn of air that a 408 requires to build real hp. They are also a 6500 rpm engine, turning them any harder over a long period will end up hurting the bottom end. Because of the added cubes, the Vjr heads and intake will work better at lower revs than they will on a 351. You would need to turn a 531 7500 to make the same power and it will never produce the same torque.