The originals were fiberboard with fiberglass insulation on the side facing the firewall. I think neoprene would sag unless it has some sort of reinforcement in it. I believe that the pad was stuck on with some sort adhesive as well as the pins going thru the sheetmetal. I scraped some of that adhesive off the inside my firewall before getting it sandblasted.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.