Will a 2001 3.8L mustang fit in 2000 convertible?

No, a 2001 3.8L Mustang engine will not directly fit into a 2000 Mustang convertible (or any other year Mustang convertible for that matter). While both are 3.8L engines, there are differences in mounting points, accessories, and potentially even the engine block itself between model years that prevent a direct swap. It would require significant modification, likely beyond the capabilities of the average enthusiast.