Concept. The Cougar/Thunderbird is a very different car than a Crown Vic (electrically).
Just wondering what kind of motor swap are you looking for? Are you looking to re-engineer the car's electrical system or to do as close to as possible to a "plug and play" swap? If looking for a "plug and play" swap, then one should endeavor to keep as much of the original car's systems as possible. That's why in the previous thread it was stated the motor's were interchangeable at the "long block" level.
When swapping a motor from a different car model the best way to do this is put both motor's on engine stands. Basically make the donor motor look like the original by transferring parts. Keeping the original wiring harness, ignition system, front accessory drives and so on.
I personally would not use a motor from a Police fleet car unless the price is very good. Why? Because police cars spend so much time idling that mileage is a poor indication of wear. There are TONS of 4.6's in the salvage yards. So there's no real reason to "settle" for the first motor seen. Try car-part.com for a searchable list of salvage yards in your area.
Note, police/fleet cars will be equipped with an external oil cooler. In order to fit into a Mustang it will be necessary to remove the oil cooler and swap with the Mustang version.
Note2, putting a PI motor into a car that originally came with a non-PI motor will work. However you will not get the full HP benefit of the PI motor without a custom tune.
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