* Size and Weight: The 351 Cleveland is a significantly larger and heavier engine than the stock options for Fox Mustangs. This impacts hood clearance, engine bay space, and the overall balance of the car. You'll likely need to modify or replace the hood, possibly even the core support.
* Exhaust System: The exhaust manifolds and routing will need significant modification or replacement to fit. Headers designed specifically for a 351 Cleveland in a Fox body are needed.
* Transmission: The transmission and its mounting will likely need to be adapted or replaced to work with the Cleveland's dimensions.
* Suspension: The extra weight might necessitate suspension upgrades to handle the increased load.
* Cooling System: A larger radiator and potentially an upgraded cooling system will probably be needed to effectively cool the larger engine.
* Steering: The steering components may need adjustments to clear the engine.
In short, while not impossible, it's a major undertaking that's far beyond a simple engine swap. It requires extensive fabrication, custom parts, and a solid understanding of automotive mechanics. Many opt for a smaller, more easily-installed Windsor-based engine (like a 302 or 351 Windsor) instead for this reason.
