There are several different types of "sound generators" that Model A passengers typically experience, and there are consequently different "fixes" accordingly. On sedans especially, the large expanse of metal in the rear "tub" are prone to resonate sound unless there is some material used to deaden the resulting panel vibrations. I personally prefer the Dynamat product, but years ago, I used roofing tar and fiberglass rolls to deaden the sound resonance. On open cars, I have stuffed the doors with fiberglass from rolls used to insulate houses - this allows for more of a "thud" when the doors close than the higher-pitched "clank" that open cars not having something additional inside are more prone to make. Similarly, for engine noises, the firewall insulation will help a good bit. The addition of a jute backing will also help to reduce noises coming up through the floor. I also understand from those that run overdrives that the reduced engine vibrations that result from lower engine rpm's will also aid in cabin noise reduction. And lastly. as you have pointed out, the absence of any interior panels will also significantly drown out any chances for any sustained conversation while driving - without the use of at least some type of panel over the doors, any significant noise reduction will be hard to achieve. Achieving a quieter ride will be the result of using all of these different components in order to achieve a good end-result. Good luck to in your endeavors.
Dick