* Engine Bay Size and Shape: The 1966 Mustang's engine bay is smaller and differently shaped than the Fox-body Mustang. Significant modifications will be needed, likely including firewall modifications, custom motor mounts, and potentially even modifications to the radiator support.
* Transmission: The transmission from the 1990 Mustang might not directly bolt up to the 1966 Mustang's drivetrain. You might need an adapter or a completely different transmission.
* Wiring Harness: The 1990 engine's wiring harness is designed for its own system. You'll need to adapt or replace it to work with the 1966 Mustang's electrical system. This is a complex task.
* Exhaust System: The exhaust system will need to be custom fabricated to fit the new engine and the 1966 Mustang's chassis.
* Cooling System: The 1990 engine's cooling system might not be compatible with the 1966 Mustang's radiator and other components. Modifications will likely be necessary.
* Computer Control: The 1990 5.0L likely uses computer control (Mass Airflow). This will need to be addressed, either by using a standalone engine management system or extensive rewiring and modifications.
In short, while it can be done, it's a major project requiring significant fabrication, mechanical, and electrical skills. It's not something a novice should attempt. You'll likely need considerable expertise and a well-equipped workshop. Many aftermarket parts are available to assist in this swap, but it still remains a complex task.
