Just before I pulled the truck onto the front yard as the centerpiece of the Christmas decorations, I glued some small, round discs over about 4 holes in the firewall that were letting hot and cold air through directly into the cab. I found the discs on some kind of nails at Home Depot. Slid them off the nails, painted them, and glued them on with Super glue. They were the perfect size. The cab is much more comfortable now.

Chuck
1950 Chevy 1/2 ton (all original)
1951 Chevy 1/2 ton (future streetrod)
1941 Chevy coupe
1938 Chevy coupe streetrod