After talking with both Baer and Wilwood plus dr diff, a stock drum/drum prop valve can be used no problem so no need use an adjustable on at least with the 11.75F/11R
There is no such thing as a drum/drum proportioning valve.
On a custom disc brake install, an adjustable proportion valve is a great tool to adjust brake bias. There are more factors that come into play than just the size of your brakes. Weight bias, tire size/make/differences, driving style all are factors to consider when building and tuning your brakes. You might be able to get by without a prop valve or with one of the generic ones supplied with the mass produced kits, but if you are gonna spend the dough and or effort to upgrade, why would you not want to optimize your work?