Is a proportioning valve really necessary in an all-original-spec drums (front and back), and all original diameter brake line diameters, and all original wheel cylinder piston diameters.
No harm using one and fiddling with it, but is truly necessary?
The original components were made to provide an appropriate balance/proportioning (having been designed with different front/rear brake line diameters & front/rear wheel cylinder diameters/bores).
I do not know about residual valves. I read that some dual master cylinders come with them and some do not.
