I feel the original posters concern, an important factor is the chosen master cylinder, it is critical the depth of the pushrod into the master cyl piston is deep (like over 1”).
If you used a C3 corvette master (68-70+-) for manual brakes it should be very deep, this depth keeps the pushrod safely captive in the master.
If you don’t have this deep piston well for the pushrod? ….you need it…get another master.
Yes, I chose a C3 Vette master, which was the recommended master for this application. The push rod does goes in quite a ways. There is no danger of the rod falling out of the end of the master. But that's not the issue that I'm trying to figure out.
The CPP instructions that Gord&Fran linked to is just like my installation.
Here again is where I'm baffled: since the master cylinder end of the push rod is not captured at that end, the pedal return spring is allowed to pull on the bottom of the arm, swinging it upward as far as it will go. The only thing stopping the arm will be the brake light switch arm. If that weren't there, the pedal arm would simply bash into the toeboard. Eventually, taking your foot off of the brake pedal time after time will beat up the switch arm and the paint on the toeboard. In this scenario, shortening the length of the push rod would do nothing except move the end of the rod further away from the piston (then you'd have to push the pedal further to get any braking).
I can't be the only guy on this forum that has installed a dual-circuit master cylinder conversion (without the power booster) in their AD truck. I'd just like to know what those other folks did that maybe I didn't do, need to do differently or haven't done yet. Depending on what that is, it might be easier to do the work before I put the cab back on the frame, and before I paint the cab.