I did this in my 65 Dart and 64 Valiant, and should do so in my 65 Newport. I used a booster and MC from a 99 Breeze. For drums, you don't need a proportioning valve. I put an adjustable one in my Dart since I plan to install front discs some day and it shouldn't hurt with drum-drum (adjust for no restriction to rear).
Currently, you have a 4-port junction block, which is open internally (no springs or pistons). If like my Dart, the top port from the MC is a 1/4" tube fitting. Get a 1/4 M to 3/16 F inverted flare adapter for that port if your dual MC's has a 3/16" tube for the front brakes (rear MC port). Don't go by the size of the tube nut since different size nuts are used with 3/16 tubing. Remove the rear brake tube from the block and plug the port (3/16 M inverted flare plug). That takes care of the front brake plumbing.
Get a 3/16 inverted flare union to connect your rear brake tube to a new 3/16" tube to the MC rear brakes (front port). You can find all these fittings in the bubble packs at Autozone.
This setup will give you everything the 69+ (or 68?) drum-drum cars had except for the "front-rear imbalance sense", which was built into the distribution block. That lit a warning light if the front and rear pressures varied. However, the switch often became stuck, so little benefit.
