I'm not sure if it will work with 3.5 as is, it just might... Since I entered X-Cart at 4.0x, didn't have the chance to test it against previous versions.
Please feel free to try it with 3.5 (I would strongly suggest you try it on a test server, even though this mod doesn't really have impact on existing code nor data, but better be on the ultra-safe side
) and do let me know if it worked for you.