The
trans. in your car is a C-4. The C-6
trans was used only on the big block engines which were not installed in Mustangs until the 67 model year.
Shift kits are available for your C-4. B&M makes one I know for sure. Make sure to indicate what year the
trans is because there are some minor differences between the years.
When you state "cat back carb" I don't quite know what you mean. Cats are a reference to catalitic converters which are installed in the exhaust system of newer cars for emission purposes.
The interior was an option called a "pony interior" and is a desireable addition to your car.
Horsepower in my estimate would be about 220 with a good tune. This is only a guess and subject to others opinions.
All the best, ---Chuck