If you can find some industrial-thickness aluminum foil, you can bend and crimp your own "insulator" around a square normal yellow insulation.
When I found my first one, all aged and groody looking, then seeing what it actually was, my thought was "Reynolds Wrap aluminum foil".
I suspect the Chry engineers had to make a decision, use an insulator under the manifold to attenuate lifter valley noise or harmonics thereof, which could allegedly crack the valley pan portion of the gasket, or leave that air space open to help quell "hot soak" heat conditions related to the carburetor.
Perhaps a small piece of DynaMat (or similar) on the valley pan area of the gasket could now do similar in modern times? Leaving the bulk of the air space under the manifold to let manifold heat better dissipate, aka "Air Flow" under there?
Enjoy!
CBODY67