Can you put 300 ford in 1966 mustang?

No, you can't directly put a 300 Ford engine into a 1966 Mustang without significant modifications. The 300 was a much larger and heavier engine than anything offered as a factory option in a '66 Mustang. It would require extensive fabrication work to fit the engine, transmission, and associated components into the engine bay, including modifying or replacing the firewall, possibly the frame rails, and definitely the suspension. It's a very major undertaking, far beyond a simple engine swap.