The originals had asphaltic panels (4) in each seat pan and that's about it. I would consider soundproofing sheet goods or the peel n stick roofing membrane (check other threads on this)..

Personally if you are going to the trouble of re-upholstering I would replace the foam. Also ensure the side bolster springs are in good shape. They are inexpensive.

While I did mine I sodablasted, repainted and relubed my slides
Don't sandblast them. Grit and seat slides don't mix.