Removal of 65-66 Mustang pedal assembly.

Measure the gap width between the steering wheel and the column tube. Write it down for future use. Remove master cylinder and rod that attaches to brake pedal.
Remove two bolts above master cylinder on engine side of firewall.
Remove rod from clutch pedal.
From under dash, remove two sheetmetal screws that hold the pedal support flanges to the firewall.
Remove two nuts that hold the u-shaped clamp supporting steering column to bottom of dash.
The pedals and pedal support are now totally disconnected.

Be careful!!!  The big tension spring between the clutch pedal and the pedal support spring bracket is under alot of tension. If the clutch pedal is allowed to move to top or bottom of its travel, it will move really quickly and can cut off your finger. The assembly can be removed as a unit with the spring in place, but I find it easier/safer to first remove the three vertical bolts that hold the spring bracket to the pedal support while the support is still in the car. Don't try to hold the spring. Just undo the bolts and let the spring come off under the dash.
To reinstall the spring, you must do it before reinstalling the assembly in the car. By pushing the clutch pedal forward about 6" farther than the floorboards would normally allow, the spring can be easily installed by hand. Again, once the spring is in place, the pedal is spring loaded and ready to try and cut off your finger. BE CAREFUL!!!

I find it much easier to remove/install the pedal assembly if you first remove the instrument cluster. That will allow you to sit in the driver seat and see what you are doing rather than laying on your back.
Also, if you are lucky enough to do this iob with the steering column out of the car, it will be much easier.

Before reinstalling the pedal support, be sure the studs for the u-clamp that supports the column are threaded into the pedal support's captive nuts.  They are next to impossible to get threaded in once the pedal support is in place.  Also chase the mc bolt threaded inserts in the front flanges of the pedal support with a tap to ensure they are in good condition and the bolts will thread in by hand.  Be sure the tab on the column clamp is indexed into the slot on the steering column.

Important:  The horn contact positions are determined by the position of the column tube in relation to the shaft. This gap between column tube and steering wheel can shift when the pedal support is removed. Be sure the gap is the same as when you measured it before removing the assembly.