Welcome aboard!
There was a thread in here, a few months back, about pre-1965 Chrysler Corp "full-size" platforms being part of the C-body family. Chrysler did not christen their full-size cars as "C"s until the 1965 model year, officially. In those prior times, there were the full-size, intermediate, and compact platforms. BTAIM
As to power disc brakes, there are several dynamics here. Most probably the car now has power drum brakes, but the power booster might not be adequate to run power disc brakes (which generally need more pressure to work well). One dynamic. The other dynamic is that without adding some complexity in sourcing components and such, Wilwood might be a better option. I usually like OEM-based conversions, but for some of the older vehicles, they might be the best alternative.
As to making the handling "firmer", polyurethane items might result in more road noise than is currently there. The lower control arm pivot (and front torsion bar mount area) is rubber, which can age and deform as the full weight of the front end rides there. New pivot bushings can be a prime candidate for replacement. No poly there that I know of. Might find some poly bushings for the upper control arm pivot bushings, though?
As I understand it, upgrading to the Borgeson steering gear (using a later-model Jeep gearbox?) can give the car a more modern steering feel and response. Which can also mean upgrading/replacing the "rag joint" and related items between the end of the steering column and the input side of the steering gear.
Lateral roll stiffness was more a function of the stiffness of the springs than sway bars, back then. There are some aftermarket front sway bars, but they look a bit flakey to me in how and where they mount. Helwig used to make rear sway bars for lots of vehicles, including Chrysler products back then. Might be able to adapt a '76 Cordoba rear sway bar, too.
THEN, complete the chassis with some good HD shocks (many in here like KYBs, but due to the way the front crossmember is designed, can be a bit harder to install (as to their internal gas pressure). A nice tire/wheel package is in that mix, too.
American VN-501 (Magnum 500 look-similars) in either 15x7 or 17x7 (which means "modern rubber" or some Hankook (or Nexen) 15" whitewalls (at about 32psi inflation pressure) can tie it all together nicely. Others might have some other wheels they might like better, which is fine.
Now, when the cars were new, their handling capabilities were better than anybody else offered at the time. The interesting thing is that a full-size station wagon, with the driver, is very close to 50-50 weight distribution as it sits. But as the chassis were more about ride comfort and safe, mild understeer at the limits, transient handling was not a real consideration back then. Yet the law enforcement-rated vehicles were the better ones of the bunch. Watch the chase scenes of "It's a Wild, Wild, Wild World" to see the Chrysler suspension dynamics from the earlier 1960s on display. The way those "then-new" cars handled on the old skinny-tread tires (at full cold inflation pressures) makes wonder what the 17x7 BFG "modern rubber" tires would do on them in more current times. Or even some normal 16" Michelin radials.
You've got a somewhat rare car which deserves the best care and feeding you can give it (not extravagantly so). Get it running and stopping as good as you can BEFORE doing anything to it. Polyurethane is not the "be all end all" some perceive it to be, by observation. There are a few vendors (P-S-T and ESPO Springs and Things) which have front end rebuild kits, as I recall. There are a few owners of these wagons which might have something to add, I suspect. Other than Borgeson, there are a few Chrysler-oriented steering gear rebuilders (as Steer and Gear), too. Which can rebuild your gear for a more "modern" or "high-effort" feel in it.
An unfortunate issue with older Chrysler product vehicles, other than B and A-body cars, is that some things will not be as easy to source in the parts area. Not a lot of "kits" per se, as modern cars tend to have available. Have to look for individual parts and pieces to make things work.
Take care,
CBODY67
