I got the 300# bars in my Fifth Avenue and you wouldn't believe the difference it makes. Not cheap, but totally worth it. When I swapped them in about five years ago I also changed out the KYB shocks for Bilsteins.

Before the springs the car would still dip in the corners by a pretty significant amount. This was with the cop front and rear sways, solid k frame pucks and frame ties. New T bars flattened it right out and the ride wasn't harsh at all with 15" BFG tires. After the the 18" Nittos went on you could feel the road a lot more.

Next biggest difference was a Borgeson steering box with FF tubular upper arms.

Only thing I'm changing on the suspension later is I'm going to move to the FF 1.25" front sway bar. After I put a big block in you could feel the additional weight on the nose vs the 360 from before. Front cop bar is only 1.125".