What you might consider doing is welding in some spacers into your spring pockets so you can run common coil-over springs instead of the units installed now. It will give you many more options to choose from. Just a 1" long piece of steel tubing either on the ID or OD of the spring in each pocket would suffice. Just make sure that at full droop, the spring can't fall out.
It is always difficult to make a car as light as yours ride well. Mainly that is due to the high unsprung weight in the rear axle. The rear axle and the tire/wheel assembly is very heavy in relation to the weight of the rear half of the car. Getting the spring rates right and then the shock valving right can be tricky.
If you would take a photo of the current springs on the car and provide us with an accurate measurement of the spring wire diameter, the overall spring diameter and with the photo we can determine the number of active coils I can calculate fairly precisely the spring rate. That will help you greatly in determining where to go next with spring selection. Also give us the spring free height (unisinstalled) and the height of the spring at ride height. Once we know the rate of the spring we can then easily determine how much load the spring is supporting. Finally, with the springs out of the rear end, set the car at the ride height you want and get the dimension between the spring seats. That is the final bit of info we will need to help you select a proper spring.