I used a product from the roofing industry here in the Pacific NW. It was a rubber like product with wax paper on either side. Came in a roll about 2" wide and about 1/8" thick or a little less. Placing the body tub and the inner fenders needed to be a bit precise as no wiggle room. In the ancient past I have used a product from the Car Restoration outlets called "Damp Course". A little more wiggle room. Ray's suggestion sounds good to Me.
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