I realize you have a Chevrolet, not a GMC. But some of the parts might be the same - MPBs would confirm/dispel that possibility.
This 1929-1957 MPB only show a
H-M transmission rear support for 1954 1/2 ton.
I was wrong on the Master Cylinder. The hole is bigger but there is no T-block on the 1/2 ton H-M/MC. A large diameter line goes towards the rear of the truck, to a frame cross-brace behind the transmission, to a T on the passenger side. That T sends one smaller diameter line to the front brake T and one smaller diameter line to the rear hose bracket on the frame rail.
I can take photos, if you think the 1/2 photos are of any use to you.
Let me know if you want me to send you a transmission-opening floor cover to use as a model (to mark the opening in the floor-board/toe-board, and for the accelerator pivot-attachment).
