Ideally, you run a Watts link or a PHB with either a 3-link rear suspension (like the current Mustangs) or with a torque arm suspension (like the 3rd/4th Gen F-bodies).
Adapting either a 3-link or a torque arm to an A/G-body is strictly a custom fabrication effort as far as I know. While you probably can retain the lowers, that still leaves a lot of work swapping out the uppers for whatever. I have seen exactly one set of unassembled prototype 3-link pieces, several years ago, for a kit that never made it to market.
You *can* run a WL or a PHB with the A/G-body suspension, but there are some important restrictions regarding installation and specifically roll center heights and upper arm bushings. Done right it's reported to be an improvement even if driven quite hard, done wrong it could get more than a little tailhappy on corner exit. That's really an installation & tuning issue, not a Watts link or panhard bar deficiency, but one that you do have to keep in mind.
Another option might be part of the Art Morrison triangulated 4-link kit (especially if you can get only the upper parts of the kit), though this will still require custom fabrication. The neat thing here is that this version of the triangulated (converging) 4-link suspension brings the roll center down to about where the above-mentioned Mustangs and F-body cars' rear roll centers are (a good thing). That, and less roll steer than OE A/G, which is kind of a good thing even if you aren't sure what it is. Launch bite might be a little better than OE.
Just leaving the rear suspension arrangement mostly the way it is, you can improve launch grip and slalom types of handling behavior simply by using LCA relocating brackets to lower the axle side pivots of the LCAs. This still doesn't fix the too-high rear roll center, but it's probably the best bang for the buck geometry fix that you can make back there.
You still get to do some suspension tuning with springs, shocks, and a rear sta-bar (incorrectly aka "sway" bar) of some sort.
Ultimately it comes down to wheels and tires. You'll want to go wider on the tires, and do a little research before simply blindly throwing your money at BFG or Nitto just because you recognize the names. Don't forget to go wider on the wheels to properly support them, too. Just because you probably can get 275-wide tires to mount up on 8" wide wheels does not make it a good idea.
Norm