1966 ford mustang is it possible to chang a coupe to a fastback?
A better option for that swap wouold be a 67 or 68, you can buy ALL of the sheetmetal needed for the change NEW. Either way, this is a VERY extensive job, you better find a really good body man and have a huge pile of money waiting.
The roof is cut off behind the windshield frame, and the tops of the quarters are cut off (or the entire quarter can be changed), everything in between is changed out in the process. New door glass and rear seats are needed also.