Re: Headliner insulation
I installed the plastic one you mentioned this past fall in my '57 along with the rubber retainer that's the original piece.
The area you don't want any extra fill is right at the outer parameter of the plastic headliner.
The rubber retainer is a very tight fit in some areas and adding anything else will make it tighter yet.
As a side note, I did use some heavy duty Velcro to hold the plastic headliner in place while fitting the rubber retainer in place.
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