I am sifting through the driveline section and am just now reading your post. I have a 1954 3600. I have done a Chevy S10 frame swap and am working on gauges. I have the 54 chassis and am considering selling or rebuilding it. Regardless, I don’t know if you ever resolved your question. There is a brake cable that connects both cables from the rear drums to the rod running under the cab. It makes a horse shoe pattern. There is a metal channel that the cable fits onto that allows you to adjust tension on the e brake. I don’t have any photos handy but if you still need one let me know and I’ll crawl under the truck to take one. I retained the original rod and e brake pedal and modified the connection at the tensioner mechanism by shortening the rod and replacing the return spring at the end of the cable.

1954 Chevy 3600