Stiffening the front end will generally make the car understeer. Stiffening the rear end will generally make the car oversteer.
Understeer tends to be safer on the street, but more frustrating at the autoX. Oversteer tends to be more hazardous on the street (the car likes to swap ends) but more entertaining on the autoX course.
If you're going to do one end, I suggest the front. Better to be frustrated than spinning into something large and immobile. (Don't ask me how I know.)
--DD
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Pelican Parts 914 Tech Support
A few pics of my car:
http://www.pelicanparts.com/gallery/Dave_Darling