Why dont you just upgrade the 5 1/4" to 6 1/2"? If you are worried about the sound stage, I wouldnt go any more than 6" away from the driver for tweeter placement. You might have the stock cutout underneath the dash pad for 4x6 plates. Not sure though.

The problem with switching the drivers is that the system wont be matched anymore which ide like to avoid. For the tweeters, I took apart the original tweets and used the plastic case and put in my component tweeters, it looks the same as stock. And yes there are 4x6 cut outs in the dahs too. I tried putting the tweeter in there but it sounded crappy when it reflects off of the windshield.