My general impressions, take them or leave them as you wish.
I despise the "cobble kits" that too many vendors sell, but I understand you're trying to make the best of a bad situation with what you have.
First, it sounds like you have a bad booster- if you can hear it inside the car, that's bad rear seals/diaphragm on your double diaphragm booster, as stated. I couldn't tell from your pictures, but what I could see makes me question whether there's even a check valve on that booster...
The pedal ratio does not change on Mopar brakes between power and manual, BUT that doesn't mean it's correct for whatever booster/master cylinder that is. The pushrod is a cobblemess, designed to directly connect the booster to the pedal. I don't even know how the booster is attached to the firewall, since there does not appear to be an adapter (standoff brackets) or reduction linkage between it and the firewall- since A body pedal assemblies have studs protruding through the firewall, that would mean the booster would need to be attached by nuts on the inside of the housing, which would mean booster disassembly would be needed to attach or remove it... Long story short, since it's bad, I'd ditch that booster entirely and not even try to replace it- that'll only get you back to where you started.
As for the master, whatever flavor that one is, you have no idea what the bore is or how much or little stroke is required to make it's compensating valve work correctly. It won't work without the booster anyway, since it's a 2 bolt master and your firewall is for a 4 bolt master. (Someone will mention the 4 to 2 bolt adapter, but that's for a 2 bolt Mopar master- the GM and Ford 2 bolt masters have a different bolt spread). I'd ditch that master cylinder, too.
Solutions? To retain your ten inch brakes (which should be perfectly adequate for your stated purpose), I would simply get a '67-'72 1" bore drum brake master and bolt it on. Under $40 at RA.
1968 DODGE DART 5.6L 340cid V8 Master Cylinder | Shop Now at RockAuto
Someone may mention using the newer aluminum 2 bolt master, but it has no residual valve for the front (helpful with drum brakes) and requires an adapter.
Since you have no proper pushrod, get the adjustable one from Dr. Diff.
Mopar '62 -'70 Adjustable Master Cylinder Push Rod
To plumb it in, all you need to do is run a brake line from the rear master cylinder port (front brakes) to a "T" fitting, and connect your two front brake lines to that. The front MC port would simply get connected to your adjustable proportioning valve, which can stay in place. Throw the existing distribution block away- it doesn't apply to a dual curcuit system anymore. Easy peasy punkin pie.
You'll need different fittings (a mopar master takes different sized brake fittings, or adapters are available) but it's all 3/16 line and bends easily by hand. With some careful measuring, you can just buy precut lengths to use and not even worry about cutting or flaring them yourself.
All in- under $200. You already rebuilt the rest of the system, why let that go to waste? All refreshed as you described, those 10" drums will be fine for what you want to do.
BTW, nice lookin' car.
