early a master cylinder swap

No I did not use a combo valve. I got a new distribution block from Inline Tube in MI to accomodate the separate front and rear splits. For this early system, run front to fronts and rear to rear separately; there is no proportioning valve for this conversion. They have one listed as made for the conversion along with the 2 new tubes from MC to distribution block, but in reality, it is probably the same as a later dual MC distribution block for something like a Coronet. My car is an early B body so you should research the distribution block for the A to be sure you get the right part; Inline Tube has a website and catalog.

A dual MC typically does not have 'wall' between front and rear sections but there is separate fuid chamber for F and R pistons. When you push on the rod, you push directly on the front piston in the MC; that pushes fliud to the front brakes and also pushes fluid to also push on the rear piston in the MC.

If one half of the system leaks, then there is hydraulic separation between the front chamber and rear chamber, so one section still keeps fluid. In my one time of a brake failure with a dual MC (front failure), I had a to push waaay down to get the 2 pistons to mecahnically contact each other and get the rear section to operate, but the 'good' half is still intact as a hydraulic system. And since you lose half of your brakes, you lose a LOT of stopping power.

Here is a good illustration including what happens with front or rear failures. (Note that the failure illustrations show the F and R reveresed from what we have in these Mopars.)
http://constructionmanuals.tpub.com/14273/img/14273_248_1.jpg