Proportioning valve

You need to know WHY, you would need a P-valve(Proportioning) or a C-valve(Combination), and what the words really mean.
So first-off, the definitions;
A P-valve reduces the pressure to the rear brake cylinders to a percentage of whatever pressure is being sent to the front.
A C-valve is a P-valve with a safety switch incorporated into it, that can be used to turn on a light on your dash, to tell you that one end (either front or rear) of your system has suffered a pressure loss.
Now, the reason.
There is only one reason to run a valve like this, which is to prevent the rear brakes from locking up prior to the fronts. When the rear locks up first, at speed, the rear usually steps out from behind the fronts and quickly sends you into an irrecoverable spin.
With Drum/drum brakes,and four same-sized tires; the front brake shoes are wider than the front, making them do about 85% of the braking. Ergo, the system is balanced by the engineering of the total shoe contact area, and sometimes the the balance of the W/Cs gets involved.
But when you install disc brakes on the front only;
It requires way more pedal pressure to affect the same stopping power because the self-energizing feature of drum brakes is no longer helping, and of course, that, way more pressure is also fed to the rear. And so, the rear brakes have a tendency to lock up first under heavy braking...... with four same-sized tires. And so the P-valve was invented to help prevent that.
Now; if you have different sized tires from one end of the car to the other, then you will have different proportioning needs.
Hotrods usually have big fat rear tires, so the proportioning CAN be engineered to send more pressure to the rear for even better braking , in combination with the front discs.
On my car for instance, a 68 Barracuda;
On the front I have 235/60-14s, with the KH four piston calipers. and
On the rear I have 295/50-15s, with 10x2 shoes.
I found that I could run NO PROPORTIONING at all, AND, I changed the rear w/cs to one size bigger, for more brake-force on the rear. This was done by field-testing, to make sure the back of the car stayed in the back, under the most severe braking situations.
My rear shoes now wear out faster than my front pads.

If you are running four same-sized tires, in a disc/drum system, then you NEED a P-valve, period.
If you have a light on your dash to tell you that the parking brake is on, then you can use a C-valve, and connect it to the light.
The C-valve from a 73 up disc front brake A-body will work just fine. And if you don't want to wire up the Safety switch, then don't,lol.

There is only one over-arching caveat, namely, do not let the rear brakes lock up prior to the fronts, because; when the back steps out, you only have milliseconds to avoid a spin-out.

Ok I hope that helps.