[84LX89GT]

I don't know how likely you are to find this, but you could try looking all over the internet for Crown Victoria police car springs, i believe even the older ones had a handling package with revised spring rates. You could also try looking up and seeing what years have swappable springs, so you can expand your year/model search. Also there are some aftermarket companies that can custom make springs for you, but you have to measure your old ones and ride height is pretty much a guessing game.

As for roll resistance, i believe the police models also had larger sway bars (at least in later models they did) and using polyerethane swaybar busings would help out quite a bit as well.

I'm not sure what year Crown Vics offered a handling package, but i know later models can be found with dual exhaust, better gears, and stiffer suspension.

One good place to start is by trying to find crown vic/marquis/town car sites and asking around on message boards, as i know for a fact there's people out there hot rodding their full size fords.
Also changing your wheels and tires can make a humongous difference in handling and get rid of alot of sway and oversteer.

One thing you may never solve is the remote steering feel because the steering linkages on the full size fords make the road feel (shock) change direction many times, which is why they had a steering gear/linkage system on them. You're not likely to get a whole lot of input from your steering system, and also i think bumpsteer is quite a concern on those because of the linkage and Short-Long arm suspension.

You should be able to make your car handle much better with improved suspension, but you may not want to expect too much from a vehicle meant to ride the "waves" of the road like an oceanliner.