Which master cylinder bore size to use for power disc Kelsey/Hayes

Soooooooooo, the Val & Cuda are about the same weight....so you would find the m/c size that was used on the 68 Val.....& get one that size.
When discs are used with drums, a proportioning valve is need to reduce pressure to the drums.....because discs require about 50% more line pressure than drums...& you guessed it....the drums will lock up without some method of limiting pressure to them.
A prop valve is not needed with all-drums because the braking force can be regulated by lining area & wheel cyl diam.