You can reuse the factory 6x8 plate behind the door if you get creative:
http://i24.photobucket.com/albums/c4...2/DSC00007.jpg http://i24.photobucket.com/albums/c4...2/DSC00010.jpg http://i24.photobucket.com/albums/c4...2/DSC00011.jpg http://i24.photobucket.com/albums/c4...2/DSC00012.jpg
Keep in mind, the above photos are NOT mine but they are what another Mustang owner did with his 2005 Shaker 500 speakers. I, on the other hand, tossed those plates in the trash because the prior owner of my car blew the mids in both of them. I didn't even think to carve the old drivers up and use them as mounting rings for replacements
ETA: As for pre-packaged component speakers, it is hard to beat Focal, Morel, Rainbow, Hertz, Image Dynamics, and even the Alpine SPX Pro line. I haven't heard your particular Focal speakers, but I would be willing to bet that they will sound awesome!