The Crown Victoria uses a modular Ford V8 (4.6L or possibly a 5.4L in some models). The engine bay is designed specifically for this engine family. A Chevy 350 is considerably wider and longer and has different mounting points. You would need custom motor mounts, potentially modified firewall, and a complete overhaul of the engine bay's structure. The transmission would also be incompatible and require a replacement.
In short, while *theoretically* possible with extensive fabrication and custom work, it's a massively complex and impractical project for most people. It would likely be cheaper and easier to simply find a Crown Victoria with a more powerful engine or build a different car around a 350.
