I am pretty sure the rules only require 500-word stories, not 1000-word stories. Hang on, let me go check.

Okay, here are the requirements from 2012:

1. at least 5 500-word permitted stories in that fandom, or
2. at least 7 100-word permitted stories in that fandom, or
3. some combination of the above (I.E.) 4 500-word permitted stories and 3 100-word permitted stories in that fandom

You cannot count:

1. crossovers
2. works in progress
3. any previous remixes you've written (for any remix challenge)
4. co-written stories, unless you and your co-writer have signed up together as a single participant
5. your safe story

So an 800-word story fills the requirements and you are good to go! (I mean, assuming Narnia is a qualifying fandom again this year, obviously.)