If you do not have spacers, you can not run the wheels reversed. Besides, it would be easier to address the aero concerns in the rear with wheels running in the normal configuration. If you run reversed, you'll mostly like want the kit GX has for sale.
Here are some threads from the "search" function regarding the use of spacers:
http://www.pinewoodderbyonline.com/post/dynasty-intimid8r-question-7331126?pid=1286533632&highlight=spacer
http://www.pinewoodderbyonline.com/post/wheel-spacer-question-7376110?pid=1286916702&highlight=spacer
http://www.pinewoodderbyonline.com/post/wheel-spacer-question-6400056?pid=1278682619&highlight=spacer