It depends on your rule book and it's enforcement. I never felt good about cheating but when everyone does you need an even playing field.
I believe Speedway Motors has stock replacement springs of different rates that appear original. I would suggest the best shock you can afford.
The biggest one thing that will help you the most is getting that car on a diet especially in the front end. If rules allow and you don't use it throw it out. Get it to minimum weight allowed. Next is engine set back if allowed. Try to at least get to 53% front and 53% left weight with out adding ballast behind the rear wheels. Most good cars are built on wheel scales. This means keep track of weight coming off and going on. A good handling car can make gobs of horsepower and hook it up. I've seen guys claim engines and be sadly disappointed because the car it was in went around the corner faster.
As far as picking springs and setting a car up your in for a drastic learning curve. You could get a basic ball park from someone that runs a car like yours, but you will want spring options for each wheel and wheel offset differences to choose from, and stager differences for the rear. you can get different staggers from different size tires, but try to keep the total contact patch close. 50 , 60 , 75 series 15 inch or 14 inch. This is an invaluable tool to tighten or loosen the rear on exit.
Here is another spot for some good info.
http://www.dirtracingforum.com/