I would try to avoid putting any additional weight into your car. It hurts acceleration. Avoid putting any spacers or anything between the coils of a coil spring. It will stiffen the spring up the same percentage of coils that you immobilize, but it will eventually start cracking the spring and then it will sag or break. Do not use any off the shelf type of suspension conversion kit unless it's absolutely, specifically designed for your car. Your twist beam rear axle will be light-years ahead of a generic IRS suspension that wasn't designed for anything in particular. Lightweight beam type axles aren't glamorous, but they work pretty good anyway. Especially when all they're out there for is to hold the back end of the car up.
What you should try, if possible, is put a bit larger tire on the front than on the back (ala newer FWD Pontiacs).
If you're not lifting the inside rear tire up off the ground in a corner, your rear anti-roll bar is not stiff enough. If you don't have a rear bar, get one. That is your first priority.
Also, things to try when you start running out of ideas--Try a smaller anti-roll bar in the front. Try softer springs in the front. Try removing weight from the front.
So- make life as easy as possible for the front tires and make the back tires do more work.....