Need to stiffen my rear 68 dart suspension

raising the front or rear of a car always has a negative effect on handling due to the higher center of gravity
new quality stock height 6 leaf rear springs and new front torsion bars in the .9" +or- range will help

http://www.eatonsprings.com/

I have .960 torsion bars
Custom upper control arms
nylon bushings
Modified braced lower control arms
hodgkis front sway bar
Wilwood front and rear brakes
frame connectors
torque box connectors
6 point cage welded
front frame shock tower braces
brace tieing shock towers together
REAR
Hodgkis sway bar
stock new leafs
KYB shocks

The back end of my car slides out in the turns when I am PUSHING IT trying to keep up with the fast Nissans, porches, BMW's

Not to many people care about road racing MOPARS so help is hard to find!!!
Lots of old FAST Mustangs but they don't get to excited about the DART

I SO APPRIECIATE ALL YOUR INPUT GUY'S