Here's the options I've considered...
Without carving up the door panels, component speakers (separate tweeter and mid/bass) can be mounted in the dashboard. Crutchfield has some nice 6-1/2 component systems which have surface-mount tweeters, and the mid/bass will reflect nicely off the windshield angle (had them this way in my '73). My dash-pad has a couple of good cracks in it anyway, so it wouldn't pee me off to mod the panel to hold two speakers and surface-mount the tweeters, pointed towards shoulders of "imaginary center passenger" for best acoustics.
My '78 had 6-1/2 speakers in kick-panel mounts and 5x7" or 6x8" (can't remember) in the doors. They sounded okay, but I haven't been able to find those kick-panel mounts anywhere, so I've got to assume they were borrowed from some other vehicle or custom made. I'm sure someone could make ones like these if you can't find a pre-manufactured set:
http://a248.e.akamai.net/pix.crutchfiel ... K2008.jpeg
I've seen several variations on the center-hump, but personally, I think they sound horrible. Same thing goes for ones mounted under the seat, pointed upwards/forward... they will sound very muffled and you won't be happy with them.
Mounting them in the headliner is a great idea! I do like that! And, by adding a bit of fiberglass resin to the back-side of the headliner (or use a heavier replacement board) would add some rigidity and cut down on vibration I would think....
A small amp will mount nicely under the seat in the center, and there are many 8" subs that will easily fit (boxed) under driver and passenger side. Again, your issue is where to put the main speakers....
Since I haven't fully decided what to do with mine (yet), I'll probably end up hacking the doors (going to paint in the spring anyway) and installing 6-1/2 components with tweeter recessed right below the vent window, angled towards driver/passenger, and main driver mounted where I have room (between the hinges in door panel or below panel). I may even toss in a couple of 8" subs below the seat along with a low-profile amp. If I go for 4-main-speaker set up (with or without subs), two smaller 4"-5-1/4" speakers will fit nicely in the factory speaker location (or could be added to the outer edges of the dash) with very minor modification. I just want a clean sound, not looking to "thump" or win any db contests
One thing to remember about mounting any speakers in the doors, if you use the foam baffles, the speakers will be more protected from moisture, and the sound is much "cleaner".
http://www.crutchfield.com/p_237XT65/XT ... ml?tp=2809