On an Avanti, the body to frame shims are steel. The only rubber ones are just over each muffler on either side of the X member. There should be 4 rubber shims. I am guessing that was to keep the floor pan away from the hot exhaust.
I had the body on a rotisserie. I cleaned off the old undercoat and resprayed it with 3M rubberized undercoat.undercoating.