GMC 6000 and Chevy C60 trucks often have a firewall mounted master cylinder that feeds a frame mounted slave cylinder and brake booster. These trucks had a single circuit system feeding all 4 drums. They worked ok but replacement parts are nearly impossible to source. My solution is to place a modern Ford Super Duty 11" dual diaphragm booster and large 1.5" bore master cylinder on the firewall with fabricated mounted brackets. Super Duty boosters are easier to source, and the Ford boosters are nearly the same design as GM parts. The booster setup in these kits is the same as most newer F550s would get from the factory.

These applications require a longer than normal rod length from booster to eyelet that connects to the pedal. This is part because of the factory design and part because a set of booster brackets hold it further away from the firewall. There isn't any off the shelf booster that has a long enough rod so we cut the SD booster rod, insert each end into a machined spacer, and tig weld it back together. The spacer is solid in the middle, so the machined rod does the holding and the welds don't have to support that force.

These boosters accept either 1 3/8" bore or 1 1/2" bore master cylinders. I expect that the larger 1.5" bore master is the way to go. Larger bore masters give a better pedal feel, although they do slightly reduce the fluid pressure. These systems still put out around 1,500 psi with the 1.5" bore master so they'll be plenty powerful.