I believe that most of those middle-'60s and later Chrysler speakers might be stamped to indicate 8 ohms impedance. Ideally, the ohms should match the system's other speakers. Using 4 ohm speakers in place of 8 ohm can result in greater volume levels for the same power setting with no long term damage, from what I've seen. For many years, GM used "oddball" 10 ohm speakers and one of the accepted upgrades at the time was the Jensen dual cones (which were not 10 ohm speakers). Currently, it seems that most aftermarket speakers are of the 4 ohm rating.
Somewhere in the folds of my brain, it seems that I recall that impedance is more of a damping situation which might also be related to magnetic properties? Not really resistance as ohms is, per se. On the electrical side of things, resistance is something we can read with a meter, though. Impedance is calculated?
Magnet size? That whole deal started in the later '60s. A bigger magnet can make for more forceful bass notes as it takes a large excursion of the voice coil to make those frequencies happen. Also takes more power to make forceful bass frequencies happen than the middle and higher end frequencies too--hence the need for greater power in later times as music tastes seemed to change.
Many of the modern big magnet/high power handling speakers have a minimum power requirement of sorts to effectively drive them. In earlier times, the magnets were small so the bass response was similarly weaker. The stiffness of the paper is also a factor, which is why the outer edge is pleated or has a rolled foam interface, to allow less resistance to the actions of the voice coil as the magnet does its thing. Lots of little, interrelated design items that make a speaker do what it does.
Physical magnet size? It might take some restraint, but probably buying the least expensive speaker (6x9, for example) dual cone speaker would be fine. Best Buy had some Pioneer dual cones for about $20.00/pair a few years ago. Not a huge magnet, but better frequency response than most factory speakers too--just no grill cloth on them. They are also 4 ohm speakers. If you find some front speakers to match the size you need, they'll most probably be 4 ohm speakers too as that's what's out there these days--UNLESS you can find some older Chrysler factory speakers of the size you need (if you find one of the old NOS accessory installation kits, it'll have wiring and many other things you might need too).
Some radio amps are more critical to open circuits than others, but ALL do not like direct shorts--period. Highly intolerant of them too! OUCH! Some "old school" techs would do the "spark test" to see if the radio amp was working. Even that might cause problems.
As for "phasing" the terminals? The old way to do that is with a C or D cell battery. The polarity with respect to the terminals will make the voice coil move a particular direction. Matching that movement with the color coded wiring will put things in phase. This will mean that both voice coils will move in unison and will tend to help bass response as the actions of the voice coils are the same instead of basically 180 degrees out.
Many modern replacement aftermarket speakers use terminals that have one wider than the other one. If the front side of the cables is not altered at the amp, no need to phase them. Many Chrysler speakers used a plastic terminal connector body to do the same thing (especially the stereo outer 3.5" speakers on the instrument panel). Some speakers also have a dot by the "+" terminal too, in some cases I've seen.
One reason the perceived imbalance between front/rear speakers might exist is the sheer spatial relationship of your ear to the speaker location. Being closer to the front speaker, it'll be louder at the middle of the fader travel. If you set the balance control to the middle on a modern vehicle stereo radio, all you hear is the front speakers (especially since they're door speakers). I usually have to move the balance control more toward the rear speakers, typically, to make the sound at my ear seem to be "in the middle of things" or to not loose the backside sounds. Just depends on the environment . . .and if you're using a "zonal" approach. Personally, I like to hear all of the speakers with some front/rear balance making for a better musical experience, unless there's someone in the back seat--then things would need to be more electronically even f/r.
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