I would have to suggest 6 1/2" speakers for the rear. Mount them toward the bottom of the 6X8 opening. The grill in the rear door is not centered over the opening in the door for the speaker. 6 1/2s will be pretty much directly behind the grill. I did this using Kenwood 2 ways, and I am happy with my install.
As for the 6X9s in the front. Not all 6X9s will fit in the front doors! I tried to install an old set of Sonys that I have, into the front door. Didn't work. They don't fit down into the 6x8 opening enough to allow me to put the door panel back on. I think it will all depend on the size of the "basket" on the speaker. I have JBL 5X7s 2 ways in the front. I really want to upgrade these though. I get a lot of vibration in these JBLs.

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Ted
Garfield Hts.,OH
'95 SE,Duratec,MTX,fogs w/parks,Ray's THF,rear dome,tranny cocktail,minor stereo mods.

GOD BLESS AMERICA!!!

Ted
tdavidk76@hotmail.com
'95 SE • DOB-4/20/95 • every option • Champagne • Duratec • MTX.
Mods: Taylor wires • Bosch +4 • Eibach/Sachs Super Gas (half done) • rear BAT urethane bushings • SVT air box (thanx Dom!) • fogs w/parks • 885 50w fog bulbs • Ray's THF • rear dome • lighted moonroof switch(thanx again Dom!) • tranny cocktail • minor stereo mods.
Yet to install: Front Eibach/Sachs • Front BAT urethane swaybar bushings.