If you are going to use an adjustable valve. Use the standard distribution-Safety warning switch that was on all pre 71 cars. That block was used on either drum/drum or disc/drum systems.
I would recomend that. The standard lines fit the original style block perfectly and you can just cut in the the adjustable piece where you desire.
You do not want to use that combo unit with a seperate adjustable p-valve. You would just be wasting your time adding the adjustable unit, and adding imbalance to the system!