Possibly ONE reason you didn't come up with much is that torsion bar/rear leaf spring suspension vehicles take LOTS of changes to install air bags. Much more than a coil-spring GM or Ford vehicle might. With the Brand X and Y, you are basically replacing the coil springs with air bags, not so on these cars. It can be done, including a relatively-easy way to put a control-arm rear suspension on them, as I recall from a post on such (probably on an older version of this website or another one).
Another issue with bagging a C-body is the rear body length between the rear axle and rear bumper. That can lead to lots of "tail dragging" of the rear bumper and tail pipe(s). Which means learning how to drive across drieway entrances and aprons for best results, which can also mean putting the unit body into diagonal torsion modes, which are not good for ANY type of body construction from my experiences with 3/4 ton box trucks.
Personally, the biggest "sin" to me of doign away with the torsion bar/leaf spring factory suspension is just how it feels (not worrying about how it might look to others) driving it on the open road. Until you have spent hours behind the wheel, learning how nice and precise the controls feel, how the engine responds to minimal throttle input, and how the suspension acts with HD shocks in how it corners at "faster speeds", you can't really appreciate how good these cars are, when compared to the "blah"-feeling similar Ford and GM cars can be.
I came to realize these things as I grew up with our '66 Newport Town Sedan (which I still have), which my parents bought when it had 7100 miles on it. Then spending 4-hour stretches of time in it going back and forth to college, once a month. Learning the little things about what made Chryslers the great road cars they were designed to be in the process.
Another side issue is that adding your proposed suspension will probably NOT increase the value of the car, going the other way, instead. It might be "a rolling piece of art" nobody else wants.
Why not seek out a '67-'68 Buick LeSabre Wildcat 4-dr hardtop instead? Nicer exterior styling and easier to "bag", too. Less rear sheet metal length past the rear axle, too.
Regards,
CBODY67