You have a mentor? Is that like... your teacher or someone else (curious - because I only had my teacher... plus a few friends to read my stuff).
But still, 6400 is within the word limit, you're fine. You still have two or so months to write it.
Also, thanks! Glad you liked it. Here's something not many people know: I did a big re-write the day before it was due
You know those bolded sections with the 'clues' and dollar values? Yeah, all of that put in the day before. So you still have time! Have fun with it though, it's the most fun you'll have in your HSC, I'd say.