Can you convert a 68 mustang coupe into fastback?

No, you can't convert a 1968 Mustang coupe into a fastback. The fundamental body structures are completely different. The roofline, rear quarter panels, rear window, and even the trunk structure are unique to each body style. It would require an incredibly extensive and impractical amount of custom fabrication to attempt such a conversion—essentially building a new car from scratch, using parts of the coupe as a starting point. It's simply not feasible or cost-effective.