I began to replace my Gas Pedal in my 56 with a Lokar one and found a switch under the pedal. This is a Automatic, Custom Cab truck. Any idea what that switch was for. Assuming is was factory.

 

If it is a small push type switch, the only one I have seen under a gas pedal is for the overdrive kickdown. I don't know if it could be for the kickdown on your automatic or not, usually they are controlled by the throttle linkage. I have seen some wired by people to shut off the ac compressor under full acceleration.
Might be able to trace the wiring from the switch and see where it goes.
Good luck
Larry

 

Don't think it was factory. You could probably use it for a NOS solenoid!

 

Does it still have wires hooked up to it?

 

In my 1960 shop manual, there's either an overdrive switch or an accelerator stop in that spot.

"Conventional Drive Transmission: Adjust accelerator stop against pedal at wide open throttle after idle adjustment.
Overdrive Transmission: Pedal assembly should just make contact with overdrive switch at full throttle."

 

a friend of mine has a 1956 Buick Roadmaster w/nailhead when you start it just turn on the key and press the gas pedal; there is a starter button under the gas pedal. Maybe the PO put one on your truck. I also remember seeing one on my Uncle's 48-50 something chevy truck.