Posted by Mopar Mitch
Aluminum notoriously has flatness and porosity problems... fiber-faced always is the better choice to give allowance for flatness and seal porosity issues.
ALSO... most people are unaware of the core-shift in the ports of the six-pack manifolds. An embossed steel gasket may, depending on the core shift of the ports -- as cast and finished from the factory -- allow the raised embossing to fall inside an intake port, in particular at the valley area, but anywhere possible... and then have sealing vacuum problems at that area. I've witnessed this problem on several six-pack manifolds ... especially the factory original manifolds... but same can be present on the MOPAR-Edelbrock aftermarket castings.
So... best to use the fiber-faced gaskets... (example being the Fel-Pro MS 90109 gaskets (fiber faced... but with embossing)... or better yet... their PERFORMANCE RACE gaskets (and trim open the center exhaust cross-over ports if you desire).
I'm a former Fel-Pro engineer from their aftermarket division... I know the history very specifically on this subject.
