The Ford service manuel says, "To correct sag or crown in the side rails the top should be up and locked and all the windows raised. then, loosen the three cap screws at the balance link adjusting bracket. Now turn the set screw counter-clockwise to correct sag in the side rail, or clockwise to correct crown. Tighten cap screws securely after making this adjustment."
The adjusting bracket is accessible with the seat and quarter panel removed. It is where the top folds into it's recess. The set screw is behind the sheet metal, and takes an allen wrench.