The extra fiber side gaskets were originally introduced as OE by MaMopar ONLY for the '69 aluminum 440-6pack intake mfld... this installed at the factory -- top and bottom of the metal valley pan. The purpose of the extra fiber pcs was to compensate for the probability of slight warpage with the aluminum mfld-- charachteristic of any/all aluminum mflds.
The side face fiber gaskets (technically called "Supplemental" gaskets) is advised, and can be used, as extra insurance for all BB Mopar valley pan gaskets/intakes... in consideration of age (warping, corrsion, etc) and block/head resurfacing... to also bring back some correct geometry.
For a true restoration, only the '69 440-6 aluminum intakes recvd these supplemental fiber side face gaskets used in conjunction with the metal valley pan gasket.
Also, because of the design of the factory intake mfds, there needs to be some type of intake "valley" plate (ie: pan gasket) to cover the area.... so, a 1-pc valley pan gasket design. This is common even with many other older big/wide engines. Pontiac V8s use a separate valley pan, and separate side gaskets.
(When I worked at Fel-Pro from 1976~1998 as an engineer, I was the lead tech-support person when we established a formal group... gave hundreds of gasket seminars across the country, wrote all the gasket set instructions, etc). I stayed with Fel-Pro during the transition when Federal-Mogul bought them out in 1998... through 2006... working as an aftermarket tech/sales rep for FM. Feel free to contact me with any gasket questions... I enjoy these discussions.