You may have to get creative with the tweeters. I don't know how the stock are mounted in yours, but often times aftermarket tweeters just don't line up with the stock configuration. My GTO was an exception...the bracket holding the stock attached to the screw hole in the center of the magnet and my MB Quarts fit it perfectly. My Regal on the other hand required a bit of surgery to the door panel for it to fit. Luckily most aftermarket kits come with a wide variety of mounting hardware and it is usually enough to figure out something that will work.
I can relate to your apprehension about drilling holes through the floor, but so long as you plan your spot out carefully, you should be OK. I usually get under the car and scope out the general area of where I am thinking of mounting it and insure there is nothing in the way that could cause problems. Just don't skimp when it comes to finding a good ground...it is essential to making sure your system has good, clean current.
One thing you may want to consider adding to your system based on the amount of power it sounds like you are planning on running is a capacitor. This will insure you have a steady flow of power when you're rocking your socks off and the amp is drawing a heavy load. They are relatively cheap, and you can get a distribution block that mounts right up to it. Here is a snap of how mine is currently setup...no show stopper by any means, but functionally speaking it works good. Be wary that although the back seat has a lot of space on it, the opening to the trunk is kind of small, so it turns out you only have so much usable room on the seat to work with and still allow it to completely fold back up, hence the cramped proximity of my cap to the amp. Of course you have a different model car than mine, so maybe you have more room to work with.