The dip around 4000+rpms is typical for a SBC with a large overlap cam. The large overlap can cause higher pressures in the intake manifold that can reduce the signal to the carburetor and cut fuel flow. Your BSFC backs up the theory. If you can adjust your carb to fatten up that area only you may get back 15-20ftlbs. With EFI it's a lot easier to get it back.
What are you cam specs?