One potential risk, too much pedal travel before the brakes engage. Leakage past the cups.
Keep in mind, that the 1 inch master is not what you want for a disc/drum set up. You will need the larger bore master for the disc/drum set up.
Using a combination valve, I don't know what that might do in a drum drum set up as its not needed. But my gut tells me that it would not be on the positive side of the equation. So little changes adding up to larger outcomes. Skipping the residual valves and then using a combination valve i would think would be a negative outcome. Maybe not a huge negative outcome but negative just the same.
Btw, if you want some good brake shoes, contact Muscle Car Brakes and see if they have something for you in their offerings. Their brake shoes are a ceramic matrix friction material as opposed to what's on the market today in terms of organic friction material. Organic friction material is not as good as the asbestos based friction material used when these vehicles were new or for year s after and the organic material just does not do the job. Not inexpensive but they work great. I have them on two of my vintage vehicles.
