problem with multipass bleeding

I''m sure someone out there amungst the heavens knowns the very simple and easy solution to this problem...I''m using multipass multitexturing to add some static shadows to project, to be more specific I''ve drawn a stone archway onto a wall with an overhang above. The stones for the arch were all drawn individually along with the wall above them, what i''m tring to do is add a shadow, across the top that drops just under the top of the arch...when I draw the shadow it does a nice job of following the curve of the arch but it bleeds horribly. I''ve tryed some of the suggestions I''ve found by searching through the forums, like Polygon Offset, reseting the z-buffer higher, adjusting the near/far clipping planes, the only one I''ve found that works is to turn off the depth test, but then my shadow comes out as a rectangle and no longer follows the arch. any suggestions?