There's a plastic plunger switch, kind of like a 1/4" diameter courtesy light switch, operated by the gas pedal, mounted way up high at the top of the pedal rod. (It's almost impossible to see or get to if you have AC ducts in there). It also operates the electric downshift solenoid in the TH400 transmission. It should work when the pedal is mashed to the floor.
If you have doubts as to it's working, turn on the key, don't start the motor, and depress the gas pedal all the way to the floor. You should hear a faint click of the door solenoid operating. There is (or,was, on my '73) a pair of wires, brown & green, I think, from the firewall down to the hood front, through a connector, and back up under the hood blanket into the door assembly. The trans downshift solenoid is in parallel, from the firewall back to the trans driver side, with pink & black (Ithink) wires.
Your plunger switch is probably siezed up, not uncommon. Reach up and push it in and let it snap back out. This will reset it. No it won't work the first, second or sixth time, but just keep snapping and snapping it and finally it will snap free. Then, when you depress the gas pedal it will automatically set itself. Some guys (myself included, before I finally got it properly reset) put a piece of rubber oil line over the gas pedal rod, but this doesn't work for long as the switch resets to this and gets stuck again.
Now I must also say, I removed my flapper door guts last year, and the trans solenoid has never worked on my car (but a new one is going in in a couple days, finally!), so I cannot say if my switch is still operating after resetting it last summer. Using a trouble light plugged into each of the the connectors I did determine the switch still worked last autumn, and that it was my downshift solenoid which was bad.