GM made a factory under floor power brake mount and had the same problem of tight space that we have. The big trucks with hydra boost used them. They used an actuator rod that was attached to the pedal ABOVE the pivot shaft so it pulled rather than pushed. On the other end is a MC bracket that mounts the master cylinder BACKWARDS and angled to the frame rail. The rod from the booster (not shown) runs to a second hole in the bellcrank which has the short push rod to the MC. Sorry but the guys had disassembled the pedals and rods with a flame wrench before this subject came back up. I think the pics are pretty graphic as to how it worked.
