I have a 70 so probably very similar to a 69. It doesn't have a fold down rear seat.
Hertz is what I have used in mine; 10" subwoofer, amp, 6x9 rear and 4x6 fronts.
The amp sits on the diff tunnel (but could go anywhere in the boot) the subwoofer sits dead centre of the parcel shelf and the 6x9 sit each side. From memory, all I did was cut the cardboard/fabric parcel shelf to fit the speakers in, the steel shelf allowed them all to fit without any cutting. I had a repro shelf so wasn't too fussed on chopping it.
The car was a factory stereo car so the door cards were already cut. The Hertz speakers were the best I found to fit the holes without fouling the door regulator. They fit exactly into the OEM holes.
I used a Kenwood head unit, bought a repro radio surround and cut it out to fit. Made up a 'box' to hold the unit from ABS and glued it all up. No need to chop the side trims on the dash as it just fits in.
Hertz was new to me but I am very happy with the sound. With a 5 channel amp I can run this low or rattle the shop windows as I go past, and the sound is clear and crisp.
My wife has a Kicker setup in her opposition product and I find it needs to be loud to pull decent sounds. The bass really bangs up loud but looses out at low levels. We bought the car with it and it has 2 amps to run the double subs and door speakers and if you run it hard the amps overload. Never had that with the Hertz stuff and it gets a hard time, especially when grunge rock comes on. The door speakers really do not perform as well as I would expect for such a name brand. In short, I could have bought Kicker but didn't.
Best thing to do is to go to a decent stereo shop with your fav CD and listen to what they have set up. Everyone's ears are different.