I think the removable part (one hole brake pedal plate) behind the brake pedal, you are talking about.
Is unique to GMC trucks, that came equipped with factory Automatic or Hydra-Matic transmissions.

This is what I think GMC did when they were assembling a truck with a Hydra-Matic transmission.
They spot welded into the cab a specially stamped out toe board with a arch to access the Hydramatic tranny flywheel and bell housing bolts.
Then, where the brake pedal is, they just cut a big Hole in the toe board.
This made it easier to lower the lower the CAB onto the chassis, over the already installed steering column.(minus steering wheel)
Then they used the removable part (one hole brake pedal plate) and bolted it onto the toe board to covered the hole.

Chevrolet does not have the removable part (one hole brake pedal plate)
picture TB 1
Picture TB 2
I think Chevrolet had a little more finesse, then GMC when they equipped their trucks with the RPO 314

I'm still trying figure out, if the toe board and firewall were one stamped out piece of metal or two pieces welded together.