Replacing/adjusting brake pedal switch?

[67Dad]

My son's 67 C10's original brake switch finally went out (we think). Symptom was that the brake lights were on all the time, eventually killing the battery. Jiggling the switch would make them go off, but not reliably.

I bought a new switch, but for the life of me cannot figure out just how I'll be able to physically get the old one out and the new one in, much less 'adjust' it. I've already taken the instrument cluster out, but the brake switch is located in a channel bolted to the firewall and there doesn't seem to be a way to reach it.

Any tricks, tips or suggestions (even if just the best way to lay upside down in the cab on the floor while working on it!) would be appreciated!

Thanks!

Mark