The valley pan is a combined valley splash shield and outer perimeter gasket design -- a common design for large wide-spread engines, such as the BB Mopars.

The fiber side rail gaskets were originally installed by Chrysler only on the rare 1969 aluminum six-pack manifolds.... due to slight out-of-flatness in the assembly. Realistically, the installation of the side fiber gaskets (2 bottom, 2 top) helps to seal the entire assembly regardless of the manifold (and head) material -- cast iron or aluminum. Best to always install them, but your mileage may vary.

I'm a former gasket engineer from a major company. I know the history behind the designs.