Howdy, I haven't seen any mention of the bias valve in this discussion. I would get one from a '67-'70 C10-30. If you can, get the little short lines, too. They are usually in good shape and are a booger to make because of all the bends in such a short distance.
On EVERY '67-'70 truck, the dual brake drum/drum system you want to duplicate, that I have seen the front pot feeds the front wheels and the back pot feeds the rear. Somewhere in your system near the front is a "T" fitting in the brake line that splits the front and back. Carefully remove it and replace it with a connector, being sure that both fronts are together and the rear is separate. Extend the rear line to the back of the bias valve, being careful to clamp the new line in with the old line so there is no chance of vibration. Use line the same diameter as the rear line that is already on the truck. That's it! Bleed system.
It really is an easy system upgrade, but sometimes folks who have done it don't realize how daunting brakes can be to a beginner. Soon you will be teaching others!
best wishes,
Les
