They won't. The way the dude ^ made those 5.25" ones fit is by just screwing through the metal with some self-tapping screws. Except it doesn't look like he used self tapping screws, haha. Ghetto stereo ftw.
You're going to need the oem adapter piece.
About the speakers, don't get those, they are garbage. If you want to get some speakers on the cheap, get some Sony Xplods from WalMart. They sound OKAY. I'm pretty sure they have the Boss brand there too if you want to be even cheaper. In the picture I posted, I was running some Xplod 6.5" up front and 6x9 in the back, off an Alpine deck, no amp. Sounded decent for the $60 or so I paid for all four. Didn't really distort until you got them up really loud.