70Tom, your car is a 1970 Fury two-door hardtop without vent windows, right?
The mirror is the original one from the car? If the mirror is from a car with vent windows (read: from a four-door w/ vent windows) its cables would be shorter because the portion of the vent window made the moving side glass smaller, and so the cables were just as long as they needed to be. Therefore such a (four-door) mirror would not fit.
You need a mirror for two-door HT's w/o vent windows, those cables are the correct length. 1971 ditched vent windows on four-doors, but those mirrors are square, so you can't use them on a 1970 model without raising some connaisseur's eyebrows.