The shaker500 doors have an additional insert that the shaker subs screw into from the front and the back to give them more structural support and reduce vibration. If you used the stock door, you would have to fabricate some sort of similar structural support or you would get terrible vibration and likely very bad sound coming from the speaker.
Even WITH the stock shaker500/1000 system you end up with a lot of vibration in the door. I'm in the process of tearing all the panels off mine right now to see what I can do to reduce it.
2005 Sport-Utility Mustang Convertible