The park brake cable does not go through the dash panel (aka firewall). The speedometer cable does have a grommet, B-14567-A, where it passes through the dash. Attached is a file from 1937sedandelivery showing the park brake cable routing, thanks Gregg.
Ps: Back when I put a small block Ford OHV engine in my car, many years ago, I put both cables through the hole in the lower left corner of the dash. I had to cut the opening to get collar of the brake cable through with the speedo cable already there.