Basically, stiffen the springs, take flex out of the control arms, different shocks, tighter front bushings, and rear sway bar.
In order to understand why the Chrysler front suspension works so well, you'll need to find the Chrysler MasterTech book on front suspensions. NOTE the camber angles of the front wheels in a turn! THAT's the secret that Ford and GM didn't know back then!
The OTHER thing is that these designs were done when tires had much narrower tread than in even the '70s. Yet the Chrysler geometry kept all of the tread "flat against the road surface" rather than "riding on the outer tread ribs or sidewalls" in a heavy turn.
Key point is that too wide of a tread width, combined with the Chrysler camber pattern, can result in only the outer tread ribs really working in a corner as the insides don't. As with the Gen 2 GM F-body cars, you can somewhat replicate the Chrysler camber pattern by adding max caster, which gets the wheels' camber in the same orientation, somewhat. Then limit lean in the corner and things work well. Otherwise, even a P225/70R-15 will wear the outside tread ribs first.
Read the MasterTech course, look at the pictures, and consider the dynamics when compared to modern tires, and I think you'll see why "the widest" isn't always the best. They'll look "killer", but actual performance might not be any better than a narrower treaded tire.
I know that some advocate for polyurethane suspension bushings and such, but also consider that the car is designed around a certain amount of "flex" being in certain areas of the body/chassis. When you firm-up and minimize suspension flex in that area, those additional forces will be transferred to a place in the platform where they might not have been desired to be. Hence, taking flex out of one place means it will end up somewhere else, which might cause some durability issues later on. Metal fatigue, as one concern.
As for moving the rear leaf springs form their original location . . . might be easier said than done. Have to cut and re-weld the saddles on the axle shaft. Get them "off" and the pinion angle can be changed, which can also result in driveline harmonics that didn't used to be there. Vibrations/shudders, for example.
As for rear sway bars, the B-body/Cordoba rear bars can work. One friend used that on his '65 Polara 2-dr hardtop. Minor bending on the ends for the mounts to line up with the frame rails. Police models had a 7/8" rear bar, I believe? That was in addition to the stock front bar.
The OTHER thing not mentioned is upgrading the OEM steering gear to a later-model gear, ala Borgeson kit.
The Chrysler UniBody provides a stiffer platform so body flex, per se, doesn't have to be considered as much in suspension tuning (which a CAR LIFE road test on a '65 Belvedere noted). Yet putting "forces" where they weren't designed to be is not good. Making things firmer with rubber bushings which are harder than stock will be good. But going totally urethane, on a street car, might not be quite so good, in the long run.
Back when the '66s were "used cars", I determined that all a Chrysler really needed was a set of good HD shocks, some 70-series tires, and adjust the tire pressure to 30/28 f/r, or maybe 2psi more. With the stock car, if you got factory a/c, you already had the HD front torsion bars. The different tire pressure bias helped balance out the tires so that each axle's tires supported similar relative weights, for more neutral handling (which was already pretty good). The HD shocks kept everything firmer and stable. If you perceive you need more G-forces, get some radials. A car that was comfortable and FUN to drive, with just a little finessing.
CBODY67
Just some thoughts,
