I got 5.25" speakers in mine, but I can't see how 6.5's wouldn't work the same way. To keep the stock enclosures, I disconnected the tweeter leads and removed the 4" speaker. This way I could use the leads to connect my 5.25's, and the 4" won't get in the way of the new speaker's magnet structure. I simply placed the new larger speakers on top of the foam padding, and carefully mounted them using coarse thread screws into the foam. Then I sealed up the rest of the opening with a couple of layers of 2" gaffer's tape.
It may sound a little ghetto for the meticulous, but believe me, they stay put and sound great. I don't think the installation needs to be complicated, as long as your rears are just for rear fill. But if you're gonna use drivers that will move a lot of air, I definitely wouldn't recommend this.