Putting a bit of thinking into it, I have found that I have used 2 Right hand side cables ACDELCO 18P394.
To connect them together, I have used 1 of these thingies ACDELCO 15714661. You can see these parts on Rockauto. Then to connect them to the front Dodge cable, I have used a small junkyard part from the Dodge 1/2 ton...
Well, I do not remember the details, it was a few years ago, and I never had to touch the set-up since, but here goes: I ended up using 2 of the rear C4 cables, the same on both sides; I do not remember which one, but they were easy to connect under the truck, using a small steel part to connect them end-to-end (could be a C4 part, but I do not remember) to make kind of a V-shaped cable. To this, I have connected a cable from a Dodge 1/2 ton (found at the junk yard), connected inside the cab to a pedal from the same truck. One problem I had was that I had routed the cables through the frame, using the original '57 truck brackets, but when I lift the rear of the truck and the wheels hang, it pulls on the cables and blocks the wheels. Independent suspension makes the wheels hang lower than live axle suspension. As you see, I have not used any aftermarket part, but it works! Hope this helps!
I saw a post you made back in '14 about trying to figure out how to hookup the parking brake on a c4 suspension. I have the same issue as I just put a c4 suspension under my 57 chevy truck. Did you ever get this all figured out? Any help would be greatly appreciated.