I have a question for you.. in your car, for your stereo, what size speakers are in the dash and behind the kick plate??
Regarding how I did my speakers,
I think I installed Sony 4″ x 6″ two way speakers behind the kick plates . I think that the dash speakers were 4″x4″. It could have been the other way around. If in doubt, measure the openings and sketch them out, bring the sketch to the stereo store to make sure the frame of the speakers you are thinking of buying overlaps to bridge the openings. Some circular speakers may fit better than the oval ones.
For the kickpanels, you can drill into the frame around the opening, and with rubber washers as vibration mounts, screw the speakers directly into the opening frame. While you are in that area, I suggest that you check to
make sure that the tiny drain holes that drain the water that comes in there from down the windshield posts has somewhere to go. If you notice water accumulated in the floor of your car under your feet after a drive in a heavy rain, it’s because the drains are plugged and the space behind the kick panels has over flowed. I also replaced my upper speakers with modern two ways, and had to fabricate under dash mounts using a stiff type of suspension strap as a frame to attach (suspend) them under the dash.
Old cars can produce tons of ignition noise. If you have not done so already, you should use resistor spark plugs, and supression plug wires, a choke coil filter in the power line to the radio, a coil filter for the alternator, and check that you ignition shielding is in place. Ignition noise supression kits can be found at Radio Shack as well as Ecklers. The Custom Auto Sound radio that I have is a good 40 watt unit from Ecklers, that I found also has excellent noise supression qualities. Having said all this, I still could never hear the stereo with the windows down on the highway!
One cool set up I have seen for rear speakers, is to mount them in the rear storage compartments. I have seen one mount where the guy cut out openings in the storage compartment lid doors, for the speakers to shoot through, with the wires run underneath. The speakers were screwed directly into the lids. The original carpeting was left on as speaker covers, wiring run under the carpet and door sill, and the whole installation was invisible. Clearance for the battery should be checked carefully.