The usual disc conversion uses a 8" dual diaphragm booster, a Corvette MC, proportioning valve, and the mounting bracket kit.
You could get this set up and use a non disc MC and convert to the disc MC later when you do the disc conversion. The basic plumbing will remain the same.
Fred

1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine