chrispy1202 was kind enough to email me some pics of his Acerbis 6.6 on his 2003 wr450f. He said it was a direct bolt on, with no mods...So this proves that the acerbis tank that is advertised to fit 05-06 ALSO fits 03-04 with no modifications.
I uploaded the pics:
I also fitted an '04 wr450 with this tank (a friend's bike) in may. There was no modification required. It's a great tank