But...HOLD ON!!! I have been doing some checking and this is interesting:
Back in 1965/66 a company called Beverly Hills Mustang LTD (9280 Alden Drive Beverly Hills, California) custombuilt some Mustangs into Mustero's, max. 50 were probably made. The company was authorized by Ford to change the body parts as well as engine, transmission and exhaust parts. The modifications were priced at about $6500, an unbelievable sum, as you could buy a Shelby Cobra for this amount. There are brochures of that company.
ALSO...
Some Mustang stationwagons were even built by the Ford design studio, but never made it for production. In 1965 Barney Clark and Bob Cumberford, both writers for Car and Driver magazine, had a better idea. Bob, being a designer as well draw a nice looking Mustang wagon on the board and they teamed up with Bob's friend Jim Licata for financial support to realize their idea. A 1964 289ci engine equipped Mustang coupe was sent over to Italy in March 1965 to a company called INTERMECCANICA. Bob had alreday worked with this company on the Griffith, and Frank Reisner was the guy to lead the new project. They cut off the roof and fabricated a new one, the rear seat was altered to a fold-down-version, the fuel inlet was moved to the left rear fender. The tailgate could be folded downwards and the overall workmanship was excellent, but in the end they ran out of money for additional modifications in the trunk area. In February 1966 it came back to New York and was an instant hit on the Detroit Expressway. Even GM employees thought, Ford was working on a wagon and alarmed their management. Barney enjoyed that a lot. There are rumours that at least 3 were built, at least not more than a handful of these nice looking Mustang mutations are supposed to have been built.
Some information from Car and Driver Oct. 1964 issue.