You have a badly worn "regulator". This is the little crank thing that the handle screws onto. If you take the door panel off you'll see it's held on to the "scissors" assembly that bolts to the door and fastens to the glass.
I've discovered that though the entire assembly from a junkyard 1979-1993 Mustang won't fit AT ALL, you can drill out the three rivets and get the regulator itself off. Then it's simply a matter of using three "steel 1/8" pop rivets" from you local hardware store (you will need a pop rivet gun too) and you're in business. It worked for me on both my '67's doors. After also cleaning and re-greasing the scissors and various rollers I can now roll either window up and down with two fingers. And they STAY rolled up. Before I could roll a window up, slam the door, and the window would drop back down an inch.
Mustang restoration supply places can sell you the whole regulator and scissors assembly if you like. However they aren't too cheap even though they are made in Taiwan. I prefer lightly used genuine Ford stuff to cheaply made import stuff whenever possible.
If someone out there has factory original power windows in their 1967 Mustang
I would be VERY interested in seeing pictures of them since to my knowlege such does not exist.