Replacing single reservoir M/C with dual reservoir on four wheel drum system

The bore size of the dual m/c needs to be the same size as the single m/c. So that can be measured or obtained from a service manual. According to my Motors Manual 1960-64 Ply Val used a 1" bore m/c.
No proportioning valve needed. With drum brakes, 'proportioning' is done with the wheel cyl size & the width of the brake shoes.