Here's why:
* Transmission Length: The C6 is a considerably longer transmission than the Toploader 4-speed or the C4 automatic that were common in '66 Mustangs. This means the transmission will likely extend too far back into the frame.
* Driveshaft: You'll need a custom driveshaft, as the length will be significantly different.
* Crossmember: The existing transmission crossmember won't work. A custom crossmember will be needed to support the C6.
* Tunneling: You might need to modify or replace the floor pan, particularly around the transmission hump, to accommodate the larger C6 case.
* Exhaust: The exhaust system will need modification or replacement due to the larger transmission.
* Kickdown Linkage: Connecting the kickdown linkage from the carburetor or throttle body to the C6 will require attention.
In short, while the engine and transmission are compatible *mechanically*, fitting them into the '66 Mustang chassis is a significant undertaking requiring expertise in fabrication and automotive mechanics. It's not a simple bolt-on swap. You'll likely need welding, cutting, and potentially significant metalwork.
