I've done two dual master cylinder upgrades, both with stock drum/drum brake set ups. It's not as hard as it may seem. You could do the upgrade now with drum/drum set up, then upgrade to discs and then add your residual valve then. Just a thought.....

~ Phillip
1949 GMC Suburban - 10 year project
1952 Pontiac Chieftain Convertible straight 8 hydramatic
1945 GMC half ton truck - Driver- Sold 9-15-25
1946 Chevy COE - SOLD 1-5-26 BUH BYE CHEVY!
1959 GMC Half ton long bed NAPCO