I use peel and stick roof flashing from Lowes or Home Depot. It has adhesive on one side and aluminum on the other, with black rubbery material in between. It is similar to the high dollar Dynamat except but it is not as thick. It has worked fine for my '49.
One thing about keeping heat out of the cab is having tight felt seals on the pedals and sound grommets in the firewall. I use black RTV to fill any gaps in the grommets.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission