I saw a write-up where the guy installed two modern 4" speakers in a modified tray that fit nicely into the center speaker bracket thing.
http://nastyz28.com/threads/how-do-i...y-dash.210474/
I think when I get to working on my future-firebird project I am going that route.
I spent years on the fence about tearing into door panels for higher quality 6" modern speakers, because most jobs I've seen just really kill the awesome interior of these cars. However I don't want a car with a stereo that sounds that old, so I think I will end up doing door speakers anyways.
Two 6x9's in the rear, 6" in the doors, two 4"s in the center. Maaaaaaaaaaaaaaybe a small 6" subwoofer suspended from the package tray to stay out of the way. At this point I'll probably also have to factor in a small amp.
I am not a stereo guy whatsoever, and this hypothetical car doesn't even exist in my life yet, so I've got a while to think on it.
P.S. One of my first theories was to substitute the door speakers by stuffing them under the dash on each side of the dash....But I don't know if they have anywhere to go.