They usually wont just drop in, you may need to modify your stock brackets or make your own. There are some 6.5's that people have said drop right in, though. Here are a few ways people have done it.
[PICS] How can I fit a 6.5" speaker in a 5.25" door? - DIYMA Car Audio Forum
http://www.collectingpez.com/GP/GotPez_65upgrade.pdf