Yes there should be some sort of weather boot at the firewall. Varies with model. Some have a boot bonded to larger rectangular plate that attaches at the perimeter bolts/studs. May find oval hole in firewall too. Rotten rubber is the only constant.
Nearly 40 years ago, When I changed our first 67 B'cuda front brakes from 3X9 drums to 73 model discs I continued to use the original drum drum master until it failed, about a year later. Then a disc drum master went in. I don't recall ever installing a proportion valve in that car.
