Hang on I didn't see anybody post their tire sizes.
The P valve is in there to prevent pre-mature rear wheel lock up..... period.
The factory put four same size skinny tires on these cars, and if the rear tires were to lock up, the rear of the car usually steps out to the side, and if you are asleep, then a spin is eminent ! You kindof want to prevent that.
Most of us hotrodders run big and little tire combos; with 225 to 245s up front and 275s to 295s out back. So for us the P valve is unnecessary, we can easily achieve correct proportioning with wheel cylinder size.
However rear discs may have more power to stop than drums. And then around she might go.
So
If you have staggered tire sizes as above, then you PROBABLY don't need any proportioning. And you for sure don't want any residual valves on your car AT ALL.
If you already have a P valve installed, just leave it on there it can't do any harm. But you may, MAY, not have optimum rear braking with it in the circuit. In any case it is easy to disable it at a later date.
Or you could install an adjustable valve, and defeat the oem one.
In every case, you will have to do some testing after everything is installed, to make sure the car stops straight. Chose your test location with care.
And a final FYI, on the street, rear discs are a luxury; the drums are perfectly adequate, even for staggered-size tires.
The only advantage rear discs offer is freedom or near freedom, from fade, and quick pad changes. But as a streeter,when and where, are you gonna need to take advantage of these features.