Lol I'm not an audio guy by trade, but I like to do my own work. I have not done 6x9's but what I will say is make sure the speaker you're installing is shallow, cause there isn't a lot of room behind it. And to answer your question to the best of my knowledge, it is possible, however you'll need to make a braket for it. Basically the speaker is screwed into the speaker cover. Once you unbolt the cover from the interior, the whole speaker will come out with it. Then you have to unscrew the speaker from the cover. But if you could either widen the holes on the speaker, or make a bracket, it could feasibly bolt in.
2000 Mustang GT-Former giggle gas addict
1990 Mustang GT w/ Vortech V3
***resident gun idiot***