10W7 might be a bit deep for under the rear seats. A rear firing center console may be just the ticket if you don't mind losing the center seat bit keeping the armrest. Either that or one 13TW5 right there firing down. It will provide PLENTY of output. Check out dodgeman's build log on here, he is using all JL Audio, a three way setup in front with a 13TW5. Some very impressive things he has done.
Looking forward to your build, and of course you know there has to be pictures
Oh forgot to add, if you are willing to slice up the floor and are dead set in having the 10W7 under there, you could always cut a square out in the floor of the storage area and build the box deeper that way. Would take some major mods and some serious undercoating to protect everything from road debris and water.