What kind and brand valve springs are you running? A cheap straight spring might kill power and limit usable rpm you need to keep the motor up in the power band.
I just dynoed a very similar 408 with Airgap intake EQ heads and similar cam, made 504 up, 500 hp from 5700 to 6100. Torque was 520 at 4600. Something is definitely holding you back. If this is going to be a strip only car I would go with a 5,000 to 5200 stall converter. Get it to pull to over 6,000 so you can shift at 6,000( actual shift will be about 6200!)