I hope i'm in the right place (it said undercarriage). I purchased a new gas tank and tar paper insulator pad for my '68 and one side of the pad has a thin film like a thick sheet of saran wrap on one side, and just the tar paper on the other. Does anyone know if the "film" side goes up against the trunk pan, or down against the tank? I hope someone has come across this before. There must be a reason for the different sides. Thanks.