What type of material was behind the interior quarter panels on a 67 coupe? Was it the same as the package tray and seat divider or something else? Should I just skip that and go with dynamat, edead, etc?

 

Die-cut OEM padding is ready to install, and a lot cheaper.

 

Who carries this material? I haven't found it on any of the sites but maybe I'm not using the right name. Is it just a jute insulation pad or is it a thin foam pad?

 

Ford never used foam back then. You buy the pads ready-to-use. The pads were either jute, or jute with a heavy asphalt-like top layer. The coupe had

firewall pad
kick panel pads
under-carpet pad (plus the jute on the back of the carpet)
under rear seat pad
quarter trim panel pads
roof pads (fiberglass)
C pillar pads
package tray pad
gas tank pad

Install all those pads, and the car is pretty quiet inside.

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http://www.mustangbarn.com/catalog.pdf