I had no problem putting 6 1/2's in the doors, as for the rear 5 1/4's required minor work just drilling 4 new holes but no trimming was required and everything snaps back together perfect when you're done. I am sure 6x9's can fit there, I've even seen it done, but I have no regrets, sound quality over the stock 5" speakers is phenominal. You will be able to crank up the volume without distortion once you finish this mod. BTW I used nothing but 3way coaxials throughout, the addition of a small mid range and tweeters in all 4 corners is awsome IMO When it comes to SQ there is really only one right way "the way you like it" .