Great article Bob.
I have been investigating this for my '30 Tudor.
Courtesy of Google AI, sound deadening (soundproofing) effectiveness is primarily determined by four key factors: mass/density, decoupling, absorption, and sealing air gaps. I googled "What are the factors that affect sound deadening" and the answer seems consistent with my knowledge in this area.
Regarding the mass factor, I have found plenty of cheaper 80mil Butyl sound deadening products on Amazon that are probably good enough for a Model A. The heavier Butyl rubber products would work for the inside of the vehicle doors and panels and other products would probably work well for the floors (waterproof, closed cell foam with heat reflective backing).
Above my muffler I have already installed 1/8" Aluminum sheets underneath the floor boards to reflect that heat.