I installed a booster/MC upgrade on a 55 3600 Dually along with new shoes, cylinders, lines, & drums. We used a 8" dual diaphram booster & a dual MC to supplement the 12" brakes. It has suprisingly good brakes. The weaksness in the original system was the MC. A hydrovac is also another good option. I'm sure you will find as we did that the disc conversion can be cost prohibitive.
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