I have a 68 Barracuda With the entire brake system transplanted from a 70 Duster 340 with KH, &10"drums, same as you. The Duster had only the one valve and a 1-piece rear line.
After I put 295s on the back, (235/70-14s up front) I was able to remove the proportioning part of the Comb.Valve, to send more pressure to the rear,sooner. I changed the rear Wheel cylinders to 1/16" smaller, IIRC, and put a 15/16 M/C on it, and with a booster from a 73Dart.
This works incredibly well for an oem system on a streeter.I have never wished for more. It is however a little hard on rear shoes.