i recommend some firmer springs and shocks all round, bigger front swaybar and aftermarket rear bar, new bushings all round, maybe strut braces of you want/can make them.
my celica handles pretty well with a similar setup but does not have strut braces.
but then again my car does not have the power to compare with an 1G or 3S. the 18Rg doesnt seem to step out unless i want it to or on tight corners, but if i went turbo i would probably invest in some hillclimb tyres or at least the best street tread i can get to help with traction..