News flash. A Firebird and Thunderbird are not the same cars!
A V6 from a Ford Thunderbird will likely fit into your Mustang. Same for the Mercury Cougar. Especially if very near the same model year.
Note, if the Ford V6 is from a 2000+ or higher, then issues such as external/internal balance and intake changes come into play. Stay close to the model year of the original car.
2003 GT Convertible (sold)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.