Read 13.4 of the stock rules, it states;

same width & diameter
offset within .25" (including wheel spacers if applicable)
resultant change in track is allowed.

From this, you can use .25" spacer's, since the millenia wheels are right on the money for offset.
Without the spacer's, you'll rub on the inner fender (near the front where the lower plastic tray ties in) when you've got the wheels hard over. You should also look into using longer studs, although I'm not sure if that's required.

If you haven't seen it already, check out the autocross guide, it's pretty good.
http://www.itrca.com/FAQ/track/autox.php

If you're located in the NW, let me know and we can try and hook up at an event.