The thing that most people don't understand about the disc brake proportioning valve is that it is not merely setting a proportional relationship between the front and rear braking force. That is what a "brake bar' in a race car does. But unless you have 4-wheel disc brakes, that is not what the PV is for.
The main function of the PV in a disc/drum system is to mitigate the problem that occurs because drum brakes are "progressive" and disc brakes are "linear".
Drum brakes are designed in such a way that as the shoe contacts the drum, the rotation of the drum makes it grab tighter. So the braking action is progressive -- you apply X amount of force to the pedal, and you get X times 2 braking action (or some other proportion -- 1.5, or 2.3, etc.).
Disc brakes, on the other hand, are linear -- you apply X amount of force at the pedal, and you get that same amount of force on the pads.
So, you can see that the PV is not just sending say, 60% of the force to the front, and 40% to the back, because that would rapidly be overwhelmed by the doubling action of the drum brake. That type of "proportioning" (60/40 split) is built into the braking system design: the size of the rear wheel cylinders, the swept area of the brake linings, the diameter of the disc, weight distribution of the vehicle -- all these factors.
What the factory PV contributes is a "non-linear" adjustment of the rear (drum) braking force. As pedal pressure increases, it "tapers off" the amount that goes to the rear brakes, applying a crude "response curve" (more like a straight line with a big kink in it).
When you adjust the PV (assuming you have an adjustable type, like on the 67-72 K-H brake package), you are changing the point at which the "kink" takes effect. This is not the same thing as changing the overall braking effect from front to rear, or rear to front.
So when people tell me they have converted to front discs, didn't add a PV, and don't have any problems with their brakes, I am guessing they just haven't tried a full range of braking scenarios from soft, feathered stop to hard high-speed braking to full-on panic stop. Because without a functioning PV, you aren't going to get optimal braking performance under all these conditions.
I really don't know anything about the design of the Wilwood or other aftermarket PVs. For all I know, they may be optimized for 4-wheel disc systems.
The non-adjustable PVs in the 73-76 A-bodies should be a reasonable match for the typical conversion setup ("slider" front discs and 10" drums). This is built into the brake junction block or "safety switch" on the disc brake cars. The 67-72 disc brake cars had a separate adjustable PV in the rear brake line.
