1963 Dart clutch hit the floor and stays

Could be a few things. Bad clutch pressure plate, but "I'd bet not."

Broken fork, fork pivot, pushrod, or really, "any part" of the linkage between the pedal and the clutch

It's time to get dirty. It's time to jack it up and get under there. You can't fix it standing there and looking at it.

Yep! My answer was too short. It's time to look the whole system over for excessively worn or broken pieces. (including taking the inspection cover off for a look see at the pressure plate and disc) I have had mine stick to the floor under high rpm but I was able to pull the pedal back up. tmm