It is a real bear back there.Personally with the time involved, do it all at once is my advice. Polk audio rocks. dont bother with 4" unless speakers is the only upgrade you plan. Just modify the metal bracket under the rear interior to mount 6.5". I just cut a section out of the middle and widened the hole. Then, youll have 4 6.5" speakers. You could also just buy 4 of the same 6.5" components, mount the front tweets where the stock tweeters are, and in the back you can run the wire behind the paneling and have them hang out above the black trim above the rear door and face them forward discreetly using velcro to mount them. No holes drilled. I also second the dynamat, etc. but I didnt have the funds or time when I did mine.
Shouldn't take more than 3 hours for just the speaker swap. However, the rear speaker placement is really inferior. If you have the time, I would also build a box, dynamat, or something to get the sound out from behind the rear panel. I imagine 50% of the sound is lost in there. Its basically an free-air speaker.