Brake light switch???

The only return spring is in the master cylinder. maybe that spring has weakened but... In those cases where the master doesn't fully return due to too long pedal rod, residual pressure will drag the brakes.
Having seen a firewall flex issue in nearly new Ford trucks ( at the clutch master and preventing that pedal activated starter switch operation ) I can suggest a theoretical fault. Yes the return force in that little clutch master was quite a bit stronger. Those trucks were nearly new though. Your firewall is 50 years old. Has it slowly flexed, moved the master away from the pedal? Murphys Law says what can happen will happen.
So if I was certain the pedal to master rod was OEM correct and the correct master cylinder was in use ( different/wrong master cylinder could have a thicker mounting flange, not 1/2 inch thicker though... hummmm, scratch that ), first I would check the pedals upper pivot point. Should find a bronze bushing there. I don't know what fault condition would be caused by significant wear there ( probably same ). I know due to geometry a little change at one end is multiplied at the other end.
If I ruled out everything else leaving the only other fault to be firewall movement I would buy the adjustable pedal to master connecting rod from the aftermarket. That solution is a heck of a lot simpler than the reinforcement plate Ford had us install in large number of their trucks.