If you can fabricate and can use any spring I would make something like the old ground control coil over set up http://images.search.yahoo.com/image...fr=yfp-t-900-s
for the front and rear. It will allow you to use an Eiback slandered English race springs. Youíll also need a better shock such as konie yellow.
If you have to stay with a stock looking spring probably the H&R race springs I think they have the stiffiest rear spring around.
I would try going with a springs thatís about 40%stiffer than you have in the front. The way I have my road racing car set up is my rear spring it 25% stiffer then my front my fronts are 450ibs/in and the rears are 575lbs/in. my car is not quite neutral so you may want a bit more spring In the back. Also canber bolts in the rear may help.