Brake pedal falling

Switch position is adjustable, factory rod length is not adjustable. There is a bushing that wears causing a slight difference in adjustment.
Odds that you have the wrong rod are very slim.
The return spring is inside the master cylinder. That spring shouldn't allow the pedal to drop even with a fluid leak. Anyone who has ever bench bled a new master cylinder knows this.
Can the firewall move causing the master cylinder to locate farther away from the pedal? Yep. Firewall flex/movement was a OEM fault in a small number of Ford pickups. The clutch master would move away to the point that the clutch safety switch wouldn't close, truck wouldn't start. We service techs had to install a reinforcement plate in X number of new trucks. Their clutch master did have a much stronger return spring though. Is that relative to your 69 A-body? Probably not, unless its been wrecked or very very rusty. At the same time, firewall position to pedal perch position variances in production is why the brake lamp switch position is adjustable. Fords clutch safety switch position was not adjustable. Reinforcement plate was the only proper fix. anyway...
A spring on the pedal arm under the dash is not a proper fix. That could pull the pedal to master apart or pull the master cylinder itself apart. We saw a master cylinder pulled apart in a recent thread ( installer error ). Instant no brakes is not the goal.