Yes, FatMat is a sound deadener and not an insulator, I used it in the doors and rear quarters.
I would think, in an open car especially, the areas to insulate well would be the floor and firewall.
About 12 yrs ago when I built my Street Rod I used a product on the floor and firewall that had a foil face on one side of the insulation to reflect heat and a jute type material on the other that acts as padding under the carpet, the rest of the interior including the roof is done with the foil faced bubble wrap that was popular back then, I found it at a Home Center in 2' wide rolls for less that half of what the vendors were charging.
The vendors sell a firewall insulator for the A's that I feel is not very attractive, how can the firewall be insulated and since that area is exposed what could it be covered with?
I recall seeing a muffler heat shield at a swap meet, do they really work and is there one that holds up well, both mounting brackets on that one were cracked?
If the spray foam is an open cell type it can hold moisture similar to a sponge, another concern would be if the drain holes in the bottom of the door got covered.