In that case, stiffer would be a necessity. Squat would be a HUGE problem in a built V8 Z car.
Read more: http://www.chevyhiperformance.com/tech/chassis/suspension_steering/1002chp_how_to_pick_the_right_spring_rate/viewall.html#ixzz2lu2rgQF7
Since it seems like the majority of your power will be used going in a straight line, it seems from my understanding of the article that there may not be "too stiff" of a spring for your application. The S30 chassis isn't too heavy in relation to most other cars built to go fast straight, so it's hard to pick a good spring rate based off of what other people are using. I see lots of discussion of 1000lb springs on Chevelles, Camaro's and close to that. But those cars are also using a hell of a lot bigger motor.
https://www.cachassisworks.com/Attachments/Instructions/899-031-226_drag.pdf
Read page 5 in this PDF, they talk about weight transfer and spring rates for front suspension. In short, it basically explains that when the front end lifts, the weight is transfered onto the rear tires increaing traction, so the optimal spring rate would be far lower then the rear to assist in weight transfer. So where you may run a 750lb spring in the rear, you may run a 300lb spring in the front. (Just throwing numbers out there.)
http://www.competitionplus.com/2004_11_18/springs.html
Interestingly enough, this page suggests near the bottom that cars using a standard strut assembly will run a lower spring rate in the rear. MUCH lower then I was thinking. I've always thought drag cars should run very high spring rates to counter squat, but it seems I may have been wrong all along (not a big suspension guy, just good with google.) The chart's recommended spring rate for a 3000lb car: Approximate
Front: 250lb
Reat: 150lb