Matt your car should not ride poorly becuase of the mod's. I use Nissan Comp rear springs (flat) , I will tell you I had to change/cut the bump stops. With the Nissan Springs there was only about an inch of travel availible.It sounds like the car is bottoming out. Also I had to change the shocks - I got a pair of the Koni adjustables , the old style non gas type that you bottom and then turn to adjust. Everyone who has ever driven the car remarks how well it rides. I am not a kind driver to the suspension , I pound the curbs mercilessly with the car and it never upsets it.
Now 1200Rallycar , I'm curiuos as to under what condition your car oversteers. Are you carrying the brakes all the way into the apex or does it oversteer on constant throttle etc.
Carrol Smith's "Tune to Win" book says to always work the end that is a problem. So try soften up the rear. If the shocks are adjustable then soften them up or back off the sway bar anything over a 5/8 inch rear bar seems to make the car tail happy over 45 mph. If you have no way to soften the rear then by all means stiffen the front.

Tom