I got a 75 Hurst Olds and the trunk is lined with a carpet of sorts PLUS a foil sided liner. It appears it's kind of a after market thing and I have a correct felt liner coming.
I'm assuming the correct liner is just the pre-cut to fit liner and I should remove the foil insulation thing along with the old cut to fit liner??

 

The aftermarket liner is similar to but not quite the same as the original foam-rubber-backed liner. Original material is no longer available. There was also a full-width black rubber matt for the trunk but I believe they are correct for 76-77. AFAIK none of them came with a carpet liner.
IF the original liner is in good shape you may want to keep it. Usually, they end up sticking to the trunk pan or they get wet when the back windows leak and fall apart. (Ask me how I know.)
I was able to take one out of my 74 Supreme mostly intact but I still use the aftermarket one in the Salon. One of these days I'll take a close look to see how good it is.