Dude, you've got to stop saying "stick things places". It attracts Dean to your thread for other reasons than what you are asking about..
(sorry Dean, it's too fun)
LOL I laugh at all the comments that come from it so its all good..
Yes you can replace them with 6x9's. My cousin put them in the front doors of his 96 dodge and I put them in the rear doors of my 01 quad cab. Just have to do some cutting is all.
I kinda wish I had some time to mock up something like this with fiberglass and such. I like the look of speakers in the kick panels and a second one in the door or something.