Different cams made the differences in the hp ratings between engines with the same heads. The ram airs run into a whole host of different parts to make the hp they advertised.
My basically stock engine with 68-16 heads and aftermarket cam through my muncie made 300hp on a chasis dyno before it dropped like a rock at 4900 rpms with bad valve springs. At 10-15% loss through the drivetrain it works out to about 330 to 350hp at the flywheel. A little extra money and some good decisions on the right parts and a well built 400 should make 400hp.

I'm also in the process right now of rebuilding my 400. Again. There's definately a learning curve in the process. Hopefully this one will be better than the previous attempts. I'm already starting with better parts and I'm doing a lot more measuring and check up of my own on the machine work that I'm having done.