The 1965 GT used the 1966 instrument panel instead of the Falcon type (bar speedometer and idiot lights). The change from a 1965 to a 1966 guage panel is easy and in a number of Mustang restoration books. It can be done in just a few hours. Be sure to label all the wires as you disconnect them since it will be easier than trying to read all the wire color codes to plug into the right guage.
Be sure to drop the steering column and put rag on it to prevent scratching. And just be patient and follow the restortion instructions as to where the wires go. Off course it will confuse a lot of Mustang purists but, heck, it is your car. Have fun!