I don’t know of any molded rear seat foam kits for these years. Most rear seats of that era were covered with a burlap/listing wire mat and then a sheet of about 1-1/2 (maybe 2” ?) high density foam and then several layers of cotton batting. I think originally the foam was molded to some extent but I can’t be sure. Here is an installation video from Legendary if you haven’t seen it.
They sell a rear sear rebuilding kit that includes all of the material. When I re-did my 67 seats it looked like the original foam was molded to some extent, but was sort of gathered up around the bottom edges.
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On mine the foam was still in good condition so I laid a layer of muslin over it for reinforcing, added new cotton batting, put another layer of muslin over it (not called for by Legendary) and then the covers.
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