Wll the transmission out of a V6 mustang fit in V8 mustang?

No, a transmission from a V6 Mustang will generally not fit directly into a V8 Mustang. While both might use a similar *type* of transmission (e.g., both might be automatic, or both manual), the bellhousing (the part that connects the transmission to the engine) will be different. The V8 engine's bellhousing bolt pattern and physical dimensions will not match the V6 transmission. Therefore, you'd need significant modifications, likely including a custom bellhousing, to make it work, and it's generally not a practical or cost-effective endeavor.